C.,
I agree that you have done a wonderful thing for your baby. Every drop of breastmilk that you have given is like money in the bank for your baby. If you need to wean now, than that is what you have to do. However, have you considered that it probably is not the lactation, and possibly not even the pumping, that is making you exhausted? If that is the case, weaning your baby simply will not address your problem.
It sounds like the biggest problem is that you are not getting good night sleep. At your son's age age, waking 1-2 gtimes per night is not abnormal, especially for a baby drinking breastmilk. That being said, you can certainly start encouraging him to sleep longer. If you are willing to let your son cry it out, I have heard good things about Healthy Sleep Habits, Happy Child by Mark Weisbluth (http://tinyurl.com/6dgrvy). If you are not willing to let your son cry it out, I have had good success with The No Cry Sleep Solution by Elizabeth Pantley. (http://tinyurl.com/5dswlr)
There might also be some other things you can do to streamline your time. If you have to drop off/pick up your older kids, can you arrange to carpool with some other moms to get a few days with some free time? Do you need to streamline your housecleaning? www.flylady.com has some good ideas for getting things decluttered and cleaned faster. Can you to streamline meal preparations? I have a great cookbook called "Saving Dinner" (http://tinyurl.com/5u5vov) Our family absolutely lives out of this cookbook. It makes my grocery list and meal preparation SOOOO much faster. Can you ask your husband to help with the housework/chores? If you do this, you have to be willing to accept the way he does those chores. If you are financially able, can you hire someone to come in and clean every couple weeks, or once a month. They really are more reasonable than you might think, and it is a big help to have the heavy-duty scrubbing done. In my house, the best thing I can do for my peace of mind is to declutter. When my house is cluttered, I spend my "free" time fluttering around picking things up and moving them around, trying to dust here and wipe there, never really accomplishing anything, and getting more and more stressed out. When it is decluttered, I find that I am able to sit down and relax, just relax! FlyLady has some awesome (QUICK, and EASY TO DO) decluttering routines. They really help.
I guess the biggest advice I can give is that you have to let some things go during this season of life. Your house isn't going to be clean, the dishes probably won't all be done, the laundry doesn't need to be folded up and put away every time. Figure out what you can streamline, what you can cut and what you can live without for a little while. The best thing you can do for your family is go to be early and get some rest. I personally have a really hard time going to bed at night, because that is the only time no one is hanging off of me or wanting anything. I love it, and it's so hard to force myself to go to sleep, but I definitely feel better when I get a good night's rest.
If you are really ready to wean your baby, I say go for it. What you have done for him so far is wonderful. Parenting is and always will be a push-pull between meeting our kids needs and meeting our own needs. All any of us can do is the best we can do. Beyond that, there is nothing to feel guilty about! However, I would also look at what other things you can do to streamline your life. It is unlikely that simply eliminating pumping is going to bring everything into balance.
Best of luck,
S.
ps - I also second getting your thyroid tested. I was completely exhausted (like, worse than when I was pregnant) and SO irritable with everyone. Within a couple weeks of taking my thyroid supplement, I felt pretty much normal again!