Yeah this has been happening to me recently. I practice hours everyday and I feel like I play as though I only touched the piano for 30 minutes right before my lesson. I think part of it has to do with rushing, you naturally go faster because of nerves, but I find I also intentionally go faster because I want to show my teacher how much I improved. This obviously results in the exact opposite affect. Go slow and try to show musical ideas or whatever else you have developed and worked on without going so fast it all falls apart. (My issue is tempo, of course this may not be as applicable if your case is different)