I am so with you on this mate, since my band pulled a blindingly bad gig last summer, it bummed me out and I've not done anything with them since. It was a really important gig too, and we just blew completely. I tried to start another project, but I have just had THE longest bout of musician's block and I'm absolutely positive it stems from that one bad gig. I've kinda developed stage fright because of it too.
The musician's block is what is REALLY getting me down though, I just feel absolutely useless, musically. I've not written a decent riff or a lyric in something like 6 - 7 months. Sucks because when I write a song, and I don't want to sound too big-headed here, but I write good stuff that I want to perform and get other people into.
Besides that, I'm working a really lame job, the only good point of which is that it's in the NHS so I do hope to train up with them and move around a bit later on, but the job gets me down too. I want to do something I enjoy with my life, but due to shoddy debt problems that I've had for ages (and have not been helped by being laid off twice) I'm stuck in my house trying not to spend money.
So yeah, I kinda feel like giving up music all together at the moment too, having a proper crisis of faith in myself. :?