I'm tired of practicing a piece 3 hours a day for weeks and play it once or twice, then forget it completely.
I know your feeling as I get some students wondering the same thing. It seems all useless to learn a piece only for it to fall into disrepair and you feel like you have to start again. The thing is, that your brain has already absorbed some knowledge of how you played that piece before. You have already made connections in your brain that although may be faded and seem forgotten, once refreshed you will attain mastery much faster than if you had never experienced it before.
Keeping conscious memory observations written on your score can act as a guide to refresh your memory when you return to a piece in the future, do not think that you need to have memorized and mastered everything you have ever played, that requires that you play the pieces constantly or develop the facility to merely sight read the pieces with mastery without requiring the input it completely into your muscular memory bank.
Just don't feel that when you forget a piece you have lost everything you learned from it. Try to learn the piece again and it will be solved much faster than your first trial towards mastery. If you forget it a 2nd time, the 3rd attempt will be even faster. Most people just get disheartened at the 1st loss of memory, they give up and think everything they did was useless.