I've volunteered in the past and know the feeling of being burnt out, USED and other negative adjectives. When you start thinking in the negative, its time to move on. If you were there to help teach others on bike maintenance but the clientele are more interested in freebie repair, that's a disconnect for you. I wouldn't feel bad to leave behind. Someone will fill your shoes.

Sometimes, leaving a volunteering job feel like you are letting down the people, but it isn't. And like Catrin says, it should be a feel good thing.

Also being a woman in bike repair, guys are more willing to butt in your work as though you don't know what you are doing. I was getting really burnt out on refurbishing huffy's, rusted out mud/dirt filled bottom bracket, one-piece cranks, rusted on seat posts, goose necks...

You did lot of good and its time to enjoy your reward at volunteering (experience as a bike wrench). Finish your bikes and go ride. Enjoy your life.