I think there are times when we all experience this, as parents of difficult child.
All I can say is that you need to make time for yourself. If at all possible, make time to get away for a weekend, go out with your husband, go out with some girlfriends.
If you are caring for a parent, that's tough too. There are adult daycare centers. My mother in law is dealing with the same thing. She refuses to bring her mom to a daycare center, because she thinks her mom won't like it or won't want to go. The center isn't just for her mom....it's for her to get a day off.