This has been really tough for me as well--as you learn newer pieces, you tend to drop (without realizing it) older pieces. Sadly, some of the pieces I've forgotten are among my favorites. I find that drop-D pieces are especially likely to fall away, since re-tuning is an extra step, and hence makes them a little less likely to be brought out. Very long pieces are also tough (Legnani 19, Soros Theme and Variations), because learning them tends to be the only thing you do for extended amounts of time. I find that at any given time, I have about an hour of material that I can play without many mistakes or needing sheet music, and I haven't really been able to make that budge. To perform in a coffee house or restaurant, or even just to play in public, you actually don't need more.