I agree with Crankin, it sounds like you should stop going. There has to be some balance between what you give to them and something positive that you get back. Otherwise it brings you down and creates negativity in other parts of your life.

I would think you could even go back at some point if you want to give it another try. And you've learned so much, there are other ways to share that knowledge -- for example I learn from fellow bike club members when we're out riding and someone has a mechanical problem, and I've stopped to help strangers on the bike trail more than once.

And welcome back!