How do I politely bow out of volunteering? I already feel terrible?

It sounds like everyone else is slacking while you're stuck with all the burden. I would tell them if they don't do their part and get it in on time or whatever it is they need to do, then you can't help them anymore. It's not fair you should have to do this with no help, most people don't volunteer at all, it it stupid when people take advantage of one of the few that actually does.