>> As I work through the project however, I'll hit a depression, and freak out over it. I get really anxious and start to believe I'm incapable of finishing the project. I've pissed off/lost many clients this way, and now I'm hitting that point again with my current project.

I really want to be able to resolve this, as my current employer is a really great one, and I don't want to have to give this job up, but I'm seriously considering quitting and pursuing disability instead. <<
As mentioned before, the right meds and right dosage do help. It's rather difficult to move forward when the physical aspect has not been taken care of. As for your pattern of behavior when it comes to completing your projects, I have a few things I want to throw out here. Just food for thought. Feel free to reject it if you like. It sounds to me like you're engaging in self-defeating behavior. Perhaps you don't feel that you deserve to succeed, that you're afraid to succeed because then you have something to lose and further to fall. Perhaps your upbringing taught you to feel like having confidence in oneself is a sin and that you shouldn't even have a self.
Obviously, I'm speaking from my own personal experience. But at least I know why I sabotage myself and work on the root causes however difficult it is. It's a real struggle. At this point I now recognize usually when I'm doing that and can make myself hold back for long enough so that I can think rationally and act from a calm, logical standpoint. Otherwise I come across as nutty, obsessive and abrasive and that turns people off.
A for needing therapy again, self-improvement is a life-long process. Going into therapy once or even twice doesn't cure you. As you develop coping skills and get better at using those skills, the less you'll need professional therapy to help you deal with life. You'll still need support from family/friends/whoever to get through tough times - just like everyone else.
Just something to consider. Now for me to take my own advice...
I feel for ya. I'm currently unemployed, highly anxious to the point of imploding. My identity and self-worth are so wrapped up in work.I'm struggling to keep from sabotaging myself.