Many of us feel that way sometimes, I have even recently felt that way after a couple of extremely rough years relating to a couple of friends who have needed a lot of support of various kinds (please forgive me, I would much rather not be specific with any of this, not my stories to tell and not the place to tell them).
Then another situation calls for it (I had a big one just today, possibly even the biggest ever one off example, for the sake of an ex no less- although I was furious until I spoke to them about it all, in truth) and I couldn't imagine doing anything else.
The reward; although I feel a little dirty calling it a reward; is that when someone needed help they actually got it instead of just getting fucked over for the nth time. I don't care if my impact is limited or even meaningless, ultimately so am I, and it wasn't to me.
If you don't feel like that, that's okay, you took the step to try anyway and I genuinely respect that. You don't have to give all of yourself, take a step back and look after yourself when you need it, but the world could always do to be a softer place when you feel ready again!
Honestly, it sounds like you might be depressed, but if so I get that too. For the people you helped it got softer, it doesn't have to directly relate to ourselves to be better.