Well, if it's any consolation, if you use that as a conscious learning experience, you'll come out of it a far more effective contractor than you went in. I didn't let it discourage me, and I benefited from it in the long run.

Another suggestion: do a thorough post-mortem with each client. Go over what you did, what you learned, what you'd improve and so forth. I.e. not a retrospective of the work you did (which is also important and worth doing), but a retrospective of your business relationship with your client.