HI M.,
The first thing you need to do is contact your Dr. Talk to him or her about how you are feeling. From the sounds of it (and I am no doctor!) you are experiencing Post Partum Depression. It affects everyone differently. If left untreated it can create serious problems. I know from experience! The Dr. will also refer to you someone you can talk to.
The second thing you should do is get an attorney and file for custody. When you do that, take all the evidence you have found (phone records, plane tix, etc) with you and show the attorney. You need to protect yourself and your kids!
Do not blame yourself for your boyfriend cheating. HE IS THE ONE TO BLAME. Whether there were problems or not in the relationship there are other ways he could have handled things. Look at it this way...you didn't hold a gun to his head and tell him to cheat did you?
Just to give you an idea as to where I am coming from, here is a little history about myself. My first husband and I were married for almost 5 years. We were together for almost 8 years. About 5 years into the relationship things started changing. My husband had a child from his first marriage and we never had any kids together. Anyway it started with phone calls. Everytime I would answer...the person would hang up. Then I noticed cars driving by and turning around right after they passed the house. That's when he would make up an excuse to run to the store or run some errand. Being a trusting person I believed him and just put my suspicions aside. In the mean time our relationship was steadily getting worse. We stopped communicating and seemed to be arguing about everything.( I am so glad we didn't have kids to listen to it!! ) This went on for a long time. Once I found a pair of underwear under our bed and I knew they weren't mine. When I confronted him he swore up and down yes they were mine...he had bought them for me. Another time I found an earring in his coat pocket while doing laundry. He had put it inside a plastic baggy. When I confronted him on that, he said he found it in the driveway and thought it was mine. That's when I flipped. The earring was black hills gold and I do not wear or like black hills gold, personal preference sorry. I flushed the earring and told him I guess he was gonna have to buy her another pair. I ended up leaving. Like you though I thought I couldn't live without him. He denied everything said it was all in my imagination. I ended up going back to him and a few months later got the shock of my life. I came home from work early and literally caught him in the act. He couldn't deny it anymore! I filed for divorce the next day.
My life revolved around that man and I thought I was going to die from all the pain. It wasn't easy but I did get through it with the help of my family and friends. 3 years later I met the man of my dreams. We have been happily married for 5 years and have a beautiful little boy.
There is light at the end of the tunnel.....don't give up!!