A mother's heart

Summer break is nearing it's end for my kids.  I am trying to show them a good summer while still working my butt off at my paying job. The problem is that kids just don't understand the financial burdons of trying to raise a family on a limited budget while trying to save for what is to come when one of the parents has a chronic painful disease.
They want to go, go, go. I want to sit or sleep. I just can't seem to find the energy to do what they deserve to be doing as children. It breaks my heart to tell them they can't go somewhere or do something because we either don't have the money or I just can't find the physical strength to do it. I am letting them down!! They will carry this with them for the rest of their lives. Some day, when I am dead and gone, they will be sitting together at a holiday or something and discuss how they never had the money to do anything fun or mom never wanted to play with them.
How do I explain to them that I fear for the financial future of this family. That I can barely make it through my shifts at the hospital as it is and that as my disease progresses it will only get worse and I won't be able to work at all. How do I explain to them that we could lose everything just because I have the fortune of having a chronically painful and dibilitating disease? They should not have to understand these things. They are children!!
Many people who have chronic diseases find themselves asking "why me, this is just not fair". I went through that phase for like a day. But I am way over asking "why me" and find myself asking "why them"? My disease is just not fair to them. The fact that I have to say no to them because I am sick sucks beyond anything I could ever imagine. The pain in my heart when I have to say no to them all the time is beyond any pain this disease could ever inflict on me.
So, when I say that I wish there was a cure for this disease, it is not for me. It is for them!! They don't deserve this. This may be my karma, but it is not theirs!! This disease affects way more than just the person with the tragic luck of getting it. It affects all those that love and care for them. It breaks a mother's heart every time to have her children watch her fall apart.