Valmai's right, of course: knowing a tune well enough to lead it at a session entails a lot more than knowing it well enough to follow when someone else leads it. You almost need two tune-lists: tunes you can play completely and tunes you can play well enough to join in.

Your first few tunes may seem like hard work, but it does get easier as you get more experienced. One of the satisfying things about building up a repertoire of session tunes is that the same phrases and chord-sequences come up in different tunes, so each time you learn a tune, there are phrases which seem to fall under your fingers almost without thinking. So you actually learn new tunes more quickly as you get used to the recurrent patterns.

Marje