I don't know what you consider not being able to do much. Just caring for your parents is a LOT. You have some exceptional circumstances there, which would both add to the amount of things you have to do each day as well as to the physical and emotional exhaustion that could prevent you from doing as much as you would like.
I suggest that if you are feeling badly, make a journal of all the things you do each day, even if it seems like you aren't accomplishing anything. Particularly in the realm of caring for others (this pertains to ailing parents or spouses or small children, etc.), one can do a lot of things all day and accomplish nothing. That doesn't mean one isn't doing anything - it just is small stuff that isn't usually worth remembering at the end of the day. Another thing that can happen when caring for others is one can feel like "nothing got done" because the stuff that was done wasn't *personally* satisfying. A lot of what gets done is for the benefit of the other person and leaves no time or energy for other things.
It sounds like you're in a tough season of life right now - one that is demanding and perhaps doesn't feel rewarding at times. You are doing the right thing helping your parents. Do you have someone (a sibling, a friend, one of your parents' friends) that could help with some of the mundane things from time to time? Make sure you do try to take care of yourself by getting a break to do something fun/rewarding/relaxing. You might not get to do this as often as you'd like, but do make sure you get a chance to recharge.