Stop trying to memorize notes - right hand this, left hand that.. You wont be able to hold very much in your head. Begin to notice, say - I play E, G#, B in the left hand. Instaed of thinking "ok, Ill play E.....ummm...G#...yeah, that sounds right...err....B!" think, "ok, E Major arpeggio --> done"
It is exhausting keeping pieces part of your repertoire, so you could let them slip a little bit, just so long as you stay familiar with it. If you have a recital coming up in 2 weeks and you want one of the pieces ready, it wouldnt have slipped so far so you have to relearn EVERYTHING. You may have to take out the sheetmusic and refresh yourself, but within a day or two it would be back and fresh in your mind as if you had been working on it for the last 5 months! This is actually better for the music as well- If you play a piece constantly for months, it gets tiresome and worn out. you will enjoy playing it if you take a break from it every now and then.
I hope you find what I suggest helpful. Also, if you are playing mozart or bach, you will especially find it difficult to memorize-its just one of those things...
donjuan