I'd get the f**k away from these people, they sound like a right bunch of shits. So you're self-depreciating and, by the sounds of it, not exactly brimming over with self-confidence ? Is that a licence for people to treat you badly ? No, but it sounds like these people think so. They're bullies, pure and simple.
Being in a band is meant to be fun....find another band.
p.s Given their attitude to your contribution, I'd not bother telling this lot that your were leaving, just stop turning up.