Have you tried the water vapor cigarettes? Some people say that the act of holding a "cigarette" and/or going out to hang out with people on a smoke break makes enough of a difference in how they feel that it's not so tough.
Being off cigarettes for 3 weeks is a huge accomplishment! I'm sure your family must be very proud of you & you should be very proud of yourself. :) And know that no matter what your family is better off with you living. Even people with horrible parents are devastated when they lose their mom or dad. But you actually sound like you really care about your kids, so that would be even worse. Please do not do that to your children. Put those thoughts out of your mind. Every time you're thinking something like that, say the opposite thing out loud. For example, if you are thinking "My kids would be better off without me here" say out loud "My kids need me & I want to live for them".
Yes, it is a hard, hard fight to beat depression. First, you have to counter the negative thoughts. Then, you have to get more active and do things even when you really don't feel like doing anything. After that, you need to try to figure out what's causing you to get so down about life (medical issues, chemical imbalance, stress, etc). Hang in there, though. If you start fighting depression early & keep at it, it can be managed -- and some people even beat it altogether. Even those of us who don't can still keep up with our family, work, personal care & social obligations nearly all of the time.
Hope that helps! Hang in there.