So how'd the two of you stay together through dealing with a parent's death and Alzheimer's? That's easy - they were her problems and you provided the support. You are the one holding this together. Now that you have a problem, you expect the same type of support in return. Hey, this is supposed to be mutual, right? Only it isn't - you are the one holding it together. When you have a problem, the whole dynamic changes. She isn't strong enough to deal with your problem.
I can relate to all this, it's how it is between me and my wife. In my case, it's spiraled downhill faster because now that I'm the problem, she's literally coming apart at the seams.
I pointed out the other reasons this stuff happens mostly to try to tell you this really ain't your fault, and if you feel like you were taken advantage of, well, I'm right there with you. I feel like a chump.
We really aren't chumps though. Caring is the right thing to do. Just so few people are capable of sacrifice anymore. They care while it's convenient. This really sucks for people like us.