My personal suggestion is “don’t try to remember” – just get more and more comfortable creating ‘whatever you create in that moment’ from the leadsheet in front of you – OR … if you really want to remember – like, if you’re using a repertoire-playing exercise as a way to come up with ideas for actual composition – then videotape while you’re playing and go back and see what you did.
That would allow you to ‘experience’ in the moment and ‘capture’ later on. But only if there’s an important reason to capture.
One important reason to capture might be if you know you’ll be performing in a stressful situation and you want to have a few solid, certain ideas (or even exact measures) you can do in case you simply can’t connect to your creative self in that moment.
I know that sense of insecurity – but the beauty (for me) of this kind of playing is that it’s not about CRAFTING something (a specific, set arrangement) – but more about RIDING something – riding an idea that you can ride over and over and each time it will be a slightly different ride.
I think this kind of playing is the missing link (except it’s not missing … and doesn’t necessarily link) between the art of performing/recreating something that’s been composed and the art of composition. You’re not really creating ‘compositions’ as you do this, but you’re using compositional IDEAS along with a basic compositional STRUCTURE – to invent in the moment.