I really like Reverend Ruby, Mama Lucky, and Kerstin's replies.
I suggest going to the beach and having some quiet time, pray, sit and watch the waves come in and out. (I also have prayer time in my shower, because it's a guaranteed few minutes alone). Do think of a few of the things you're happy with and grateful for, NOT what needs to be fixed or done. Once you feel centered, take a notebook and write the things you mentioned in a list, and what you can do for each of these things. You have to make your own decision, with your husband, since we don't know your family situation, but here's what I would suggest:
1) School: you're in school more than full time. With 3 kids, that is a lot! Finish this semester (you can do anything temporarily!) and I would suggest lightening your load next semester. No REAL reason you "have to" take it all on at once. Just lighten it to a "regular" load, yes?
2) Hooray for your husband! Don't forget to congratulate and celebrate. Not just for him, but for you too.
3) 2 Jobs: 1/2 day child care and part time at a restaurant. I don't know your situation, but from my view, the day care would work a little better. You're home with your kids, could get a little housework done, could possibly cook, could possibly do some home chores (homework, phone calls, bill paying, whatever). At the restaurant, you can do nothing but restaurant stuff. You DO need to tweak your budget a little bit and change some of the spending so you don't need one of those jobs (just cut out some things--lower the minutes on your cell, eat out 1 day less, don't buy Starbucks, whatever). I would suggest keeping the job that gives you some opportunity for multitasking other things rather than the one that gives no opportunity for multitasking.
3) Baseball and dance are part of life. May I suggest carpooling or a mom taxi kind of service? My friend worked a lot and I would pick up her son from school, take him home for snack and a little bit of homework, then take him to soccer. She would pick him up towards the end of soccer (a couple times she didn't get there in time and I took him home with me).
4) Younger daughter: She thinks the world revolves around her? This might be her reaction to seeing her older sister in dance and her older brother in baseball, and she feels like she needs to be demanding because she doesn't understand "ages" and just feels left out? Try to think of a way to carve out a little time where she feels special (easier if you have 2-3 nights free on your calendar, once the restaurant job is gone). In the meantime, set boundaries and teach her that while you love her, and she is an important member of the family, the world does not revolve around her. You all work TOGETHER, as a TEAM. She's 4, so she can't control your decisions, period.
5) Older daughter: the tonsil thing is important to do, but not a bad thing. I'm sure this is just the straw that broke the overloaded camel's back. Just get that done, and keep icecream in the house.
6) For your house: I found my amazing housekeeper through this site (mamasource then)---I had a surgery and some complications, and just wanted someone to come in once a week to take care of the "stuff". When I was better, she would just come in to help me when guests were coming or I was having a party (I was busy with shopping and cooking, and she would do the cleaning to take that load off). When I was pregnant, I would do most of the weekly upkeep and she'd come in once a month to keep it great. Now that I've moved, she is our official housekeeper in between renters at that property. I love her! You could hire someone for once a week, once a month, or even just a one time thing to come help you get caught up. You declutter and get all the junk out of the way while someone cleans for you. A HUGE HUGE HUGE burden off your shoulders.
Also, I strongly agree with chore charts and everyone having a few things they do EVERY DAY to keep the house under control. I adapted stuff I learned from flylady to do what works great for us. If you want to see that, private message me and I'll send it to you, if nothing else than as a template for ideas on what you'll do yourself.
Whatever you decide to do, you CAN do it! Good luck to ya.