One of my problems is tension. It's a very common problem, exacerbated by the critical atmosphere in classical music training and performance, not to mention my own hyper-analytical attitude. The tape loop in my head is commenting: hard part is coming up, try and get it right this time, nerves on high alert, try try try... ooh, just screwed it up again. It's pretty clear that the response to the problem is part of the problem itself. There are a variety of approaches to trying to deal with this. I've mentioned some before, and Barrick Stees comments on the unsuccessful Cleveland audition are relevant too: in a high pressure situation, pretend that you're giving a lesson. My son's piano teacher suggested playing while talking, as a path to effortless playing. (It has to be something you can talk about easily, though. The teacher asked him what he had for breakfast, and he totally couldn't remember. Eventually he replied "A carrot", which seemed like an unusual breakfast to the teacher -- I don't think the passage he was playing came out very effortlessly. My wife suggested changing the topic to the properties of bismuth, something my son has been interested in, and that worked much better. Not sure how you'd do the playing while talking on bassoon, though.)
Recently I've started reading jazz pianist Kenny Werner's book Effortless Mastery, which seems to be concerned with this effect. Early on he describes his experience with a vaguely mystical teacher, this is already after he's been through Berkelee and is a working pro, who only lets him practice one note at a time. One note, one finger. Aiming for total relaxation of that finger and motion. And not as some kind of quick warm-up, before setting into hours of intense, high-tension "real work" in the practice room. Rather, no more than five minutes of this, per day, and no other playing. After some weeks of this, spending most of his time hanging out at the beach in Brazil or whatever, he went to a party, and got asked to play. After many apologies about being out of practice due to his crazy teacher, he did. And everything sounded wonderful. Much of the problem is in our heads, in particular, in the parts of our brains responsible for analytical critique. The part that decides, is it Right, or is it Wrong, the Manichean fallacy. Nonjudgmental should be the goal. "If you can meet with Triumph, and Disaster, and treat those two Imposters just the same." (Kipling) Or alternatively, in words stolen from forum, you're not as good as you think you are. You're not as bad as you think you are, either. At my lesson, we did a few things along the lines of relaxation. One was to exhale in time before inhaling, both to get a fuller breath, and also because deep breathing is a relaxation exercise. Another was to listen to the tone from the room, trying to ignore the direct sound of the instrument coming through the mouth and jawbone. This morning I spent my practice trying to figure out how to practice that mental state, how to play effortlessly and relaxed. I ended up playing scales at MM=48, which I'd just picked to be slow, but is probably a good tempo to establish a slow breathing and a slow heartbeat. Pulse is physical, and relaxation is physical.