What about those matches where you are indeed the favourite to win but things are not going your way? I've lost quite a few important ones against people I really shouldn't have trouble with. Yes, I wipe the floor with them in practice matches, but at tournaments (with about ten B class players including myself and other 40 or so C class players) it's a different story...
If you lose to them (the C guys), it's embarrasing, and if you win, it was expected of you by everybody around so it's not much of an accomplishment. After all, I do practice around 25-30 hours a week, so I'm always in stroke and fully prepared, so there can be no excuses. To make things worse, some of those C level guys have vast tournament and even gambling experience and are really mentally strong that you wouldn't believe. They may have trouble running more than 5 balls, but they don't beat themselves like I do.
On the other hand, If I'm confronted against people of my skill level or better, I generally play much more relaxed because I don't fear losing so much in that case. Win or lose, those matches are much easier to play for me.
Now I'm in the process of reading "Pleasures of small motions". There are some great ideas in there which really make sense. Hopefully the new approach will help. I won't quit.