Sounds like you've been a working mother all these years. Who took care of the baby then?
One important thing that helps when you have a baby or toddler is to have a routine. Get up at same time, eat at same time, nap at same time, go to bed at same time. Also have routines within the routine. I wonder if what would help you is if you spend some time (a month or two at least; it may take more time) to set up the household so it runs reasonably smoothly.
I know of several women who went to school and/or work and found ways to take care of their children. Your post sort of sounds like you've boxed yourself in. Think outside the box.
While you're getting your household organzied and manageable baby proof the house. Put up anything that your son could reach and damage or hurt himself with. Put child locks on all the cabinets and drawers. Lock the bathroom with a hook that is high enough that he can't reach it. Develop a routine in which your older kids watch him some of the time. A routine with one room being a playroom with a baby gate or a dutch door installed. This way your baby is out of your way, safe, learning how to self entertain and you can still hear him if there is a problem. This set up will make time for study more possible.
My daughter attended Portland Community College and she could leave her baby at the Child Care Center for a minimal fee. That would take care of class time.
My daughter also obtained loans. She could have borrowed enough for child care but she had me to do child care for her. Several years ago, at least, there were government programs available that you might be eligible for especially since you're military.
Both my daughter and I believe it's best if you don't have to use a child care center but found that sometimes you must. Day care can work if you've researched and found them to not only be reliable but to also have caretakers with whom you mesh.
My granddaughter went to a preschool sponsored by a church. She was 3 when she started and hated leaving to go to kindergarten at 5. The large group had a great indoor play area. They broke up into smaller groups by age and did quieter things in classrooms. They learned the basics; ABC's, numbers,colors etc. She had two different teachers during that time. She knew all of the teachers as well. They loved her and she loved them. WE liked it because they did include Bible stories and taught the kids manners. There are private smaller preschools such as this one held in churches that are not connected to the church in any way except that they rent space. The cost in the smaller private places is frequently less than in a large corporate day care. And the care is more relaxed and homier. No corporate rules to follow.
My grandson is in daycare in a home based day care/preschool. This caretaker is licensed for no more than 6 children under the age of 6. The kids get individual attention. My grandson has speech and anger issues that made it difficult to find the right place. When there are too many kids he's out of control. He gets along well with these kids and the caretaker. She knows how to be firm and loving at the appropriate times.
Finding the right place is time consuming. I found the home day care on the Internet. There are organizations that provide lists. The Oregon Department Of Human Services is one of them. I imagine Washington has such sites also.
I also found sites that taught me what to look for and what questions to ask when looking for daycare.
I'm sensing a bit of fear from you perhaps because this is something entirely new for you. I adopted my daughter who came to live with me at age 7. Parenting her was way different than parenting most children born to the family. So you are learning as you go with a baby. And you have two teens at the same time. I could see that you could have difficulty getting your older children to babysit especially if their first years were chaotic and perhaps abusive.
Which brought up another thought. I retired when my daughter was 13 because she needed better supervision and was too old for a child care center. In some ways, teens need more attention than younger children.
I think that if I were you I'd take time for each step that you want to accomplish. First get your house and family organized and running as smoothly as you need it to be to go back to school. Perhaps take one class by computer to provide intellectual stimulation for yourself. See how that goes.
If home isn't working in a comfortable way for you, focus on learning different home management and parenting skills before going seriously back to school.
During this initial time brainstorm for ideas as you are doing here and check them out. Take time making a choice to be sure that everyone is comfortable with the decision.
Stopping a career and becoming a SAHM is a big! adjustment especially if you enjoyed your career. I was a single mother. I found that I felt different about being home and parenting because no one else, not even a day care person, was reinforcing my values and rules. I was also not getting positive feed back from co-workers or the sense of success that work gave me. For me the support of friends was very important.
One family had my daughter to their house every Friday night so that I could have one night free just for me. Another friend took her for an hour or two or even overnight when I was frazzled or sick. If you're new to this area I recommend that you take time to make friends and build a local support system.
When looking for daycare think outside of the box. I've known of families that traded day care. Your time will be more flexible than it was working. Sometimes people find that a neighbor is delighted to care for their baby/toddler for less money than a licensed place. If it's a neighbor that you've become friends with you will have more input into his care.
In summary, slow down, work out each step before you advance to the next step. Treat it as on the job training.
My daughter did not do well being a SAHM. She had me to help and so she went back to work soon after her babies were born. She went from school to work to being mommy. She is just now, several years later, discovering that she has to do more organizing and planning ahead for her to be comfortable at home.
I'm a retired police officer. You are in the military. Being a SAHM is more difficult than either of our jobs probably were, mainly because it's so different. We have to provide the structure at home that was present for years at work.
I have a degree in Home Economics which helped me a lot. I also did a lot of reading about parenting because my daughter came to me as a special needs child. A part of what I learned while researching parenting was ways to create a workable schedule. However, I had difficulty creating structure after I retired. Baby's and children need structure in order to feel secure.
Several moms on this site have asked questions relating to being a successful SAHM. There have been questions about how to build structure and how to find time for ones self. And there have been dozens of good answers. This site's archives could be a helpful resource. Because I haven't tried looking back I don't know how easy it is to find previous answers.
A comment about the 18 month old tiring you out. I took care of both of my grandchildren at that age. I didn't have to keep running after them because their mother and I had baby proofed the apartment. I did have to be aware of where they were when it became oh, so quiet. I soon learned that I had to lock the bathroom door from the outside. :):)
I had to be available when they needed me. I could do housework but time was much easier when I was willing to stop and pay attention when they asked for attention. Sometimes all they wanted was a "pat on the head" sort of interaction. Other times I'd set them up playing something different before going back to the dishes.
Could it be that your son is asking for structure? He will be more active anytime things change and it sounds like you are in the midst of a couple of big life changes. Babies are very perceptive of the adults' emotions but they don't know what they mean. They beacome agitated and need some firm direction. Spending quiet time talking about the changes at the baby's level often helps him to calm down. And providing some physical activity preferably outdoors helps them to run off the energy their anxiety has created.
I also suspect that once you're settled and found a routine and structure that works for you you will be less tired. If I were you I would be anxious and even frightened. In earlier days I would have expended energy to keep those feelings unknown even to me. I've learned that I have more energy and can better think and make decisions when I'm able to acknowledge my own feelings and discuss them with a friend or sometimes a therapist.
I'm pulling for you! You're in one of the most difficult places to be. You'll be losing your routine and have less confidence being a SAHM than you had while working at a job you've done for years. Everything is new and you'll feel alone. One day you'll have that old confidence back.