I haven't read the whole thread thoroughly, but two things popped into my mind:

your bike doesn't fit you or has a too aggressive set-up, so it's "forcing" you into a position you're not comfortable with

you're out of your comfort zone too much. I'm a beginner at mtbiking proper, but I recognize the same feelings from rock climbing. Some people get better by pushing themselves all the time - I don't. Rock climbing, or biking, or anything that has a bit of risk involved - I do better when I've done the easy stuff a million gazillion times so that I really really really feel comfortable pushing myself a little. If I feel I'm out of my depth I tense up, over-analyze things, over-compensate and do horribly. And riding or climbing with people much better than me I feel out of my depth fast, but if I'm one of the best of the group I feel much more courageous.

If I were you I'd go out and find someone even more of a beginner to ride with!