Guess what, you have every right in the world right now not to be happy and not to up to snuff. Quit being so hard on yourself, you are after all only human. Oh, I know, husbands and kids think wives and mothers are these super human people that can do anything no matter what.
I am still surprised you are able to work with all of this going on around you. I don't remember what kind of work you do. One thing I can tell you, is pain can and will suck the life out of you. Given the fact that your body really isn't getting the rest it needs and something is wrong with it, no wonder you are miserable.
You said you are depressed, well pain and depression go hand in hand. You might consider getting something to help with the depression. You have a number of tings stacked up against you right now so you may needs a little help for while.
Now might be the time you make a "chore list" for each one of the kids and one for hubby. After surgery you are not going to be in any shape to be cleaning house, doing laundry and cooking. Just because you are having surgery does not mean an overnight quick fix to mom's health problems. Your family needs to understand this. They also need to know how important that it is that you recuperate and only do the things your dr says you can do right after your surgery. If you go home and start doing things too early you run the risk of messing up things from your surgery.
It probably would help you to take a few days off of work before your surgery and try to rest up some. I guess what I am saying is you need to worry about you and what its going to take to get you back up on your feet.