I think all of us with BPD feel this way once in a while no matter how supportive the people on this site are. Kinda like we never quite feel good enough, and don't understand why anyone would find us interesting enough to talk to, much less read our threads.
When I get like that, I have to step away from the site, and deal with my fear which usually turns into depression, if I don't catch it. The site is great for destraction but sometimes it gets to be too much and I have to step away.
Anyway, you're not alone.