I can certainly relate. I also have physical and mental issues, and sometimes I wonder the same things. The best thing to remember is that we also have an enemy who came to steal, kill, and destroy. That enemy, make no mistake, wants us miserable. He also wants it to look like God is attacking us Himself, so that we lose faith and turn away from Him. While it does seem that God is just allowing it, we need to trust (oh, do I have trouble with that myself!) that He is going to eventually take the enemy's tactics and turn them around, using them against him and to our benefit. By analogy, God is going to build us a beautiful mansion out of the very bricks Satan keeps throwing at us.
As a writer, I can look at it this way. Let's say I have a character going through some terrible things. If my characters were self-aware, they would no doubt wonder why they have to go through those things. But I know how the story is going to end, and I know I'm going to make everything work out beautifully for that character.
God is the one writing my story, and yours. He knows how it's going to turn out. We don't. But everything we go through is going to be a piece of how it all comes together.
I hope this helps. I really do understand.