It's tough to press record and bam, get a good take right out of the gate. So what I do is hit the record button before
I do a final tuning check. Then I'll noodle through something for a few seconds, maybe play a short passage from the piece I'm trying to record (recording all the while), and when I feel relaxed, take a deep breath and start playing in earnest.
If I make a false start or two, I don't worry about it and I don't
hit "stop". I just take another deep breath and start over. I'll often get several complete run throughs plus a number of false starts and stops all on one continuous track. Later, I can pick which complete take I like best -- which, oddly enough, is usually the first take.
But you can
eventually get to the point where you just have to say, "well, it's just not happening tonight," and try it again at some later point. If you are loosing concentration or inspiration, if it seems to be sounding worse rather than better, it's probably time to give it a rest.
I record on the PC, so it's very easy to just "slice" out the take I like and delete all of the other stuff, but this might be a little more tricky if you are recording to tape or to disc. In fact, if I'm really feeling lazy or frustrated, I'm not above doing the occasional "punch in" either.
Hey, I'm not proud.
Anyway, just some ideas to try to help you loosen up a little.