Can someone more advanced help me with something? I am self taught and have been playing for over 2 years now. I train in the classical way but have fun with all the other stuff like blues and jazz... Lately, I am at a point where almost all chords, modes, scales are known to me well but need a bit more regular practise. I have practised a lot for the last 2 years but not played that much music yet... I know maybe 15 easy/medium pieces by heart. When I play these pieces, sometimes I lose them and get it wrong. I know they are in my fingers and in my head but it can be like night and day!! Can anyone advise me on what's happening?
I think, like a lot of self taught players, you are learning each one verbatim. It always starts like this, goes like this in the middle, and ends like this. When you start to play with others, you'll find that they too have their own way of playing a particular piece. And often, it's not the same way as yours.
Try to compartmentalise pieces - they are often written with verses, chorus, middle 8, bridge.Change the order you play those in, and be aware of how one section morphs into another. Change them about, so you know the piece starting half way through. Use different tempos, different rhythms, just because you can.
Playing with others will make you realise that there needs to be some give and take, so you'll end up analising the pieces from a different viewpoint. At a gig last night, someone came on and played a song I've played for years. He did it 'wrong', as in it was different from the original version, leaving out a bar somewhere, and putting an extra one in somewhere else. On purpose or a mistake? Never asked. But needed to change my way to fit his, 'cos it wouldn't have worked vice versa! So, mentally, on the fly, an extra bar was put in the next time around, etc. Those who learn a piece from top to bottom tend to get lost at times like these.
Try changing the voicings on the accompaniment, try changing a chord here and there, starting by making a major into a 6th, or 7th. Listen to how it works - or doesn't! Record yourself, and play along with it, using a different octave, or bass line. Move away from the strait-jacket!
It sounds like you may be relying on muscle memory instead of "best" memory where you engage active thinking along with muscle memory. You should actively think about the "sections" of the piece, how they relate to each other, what the composer intended here, and whatnot. It helps to just sit down and look at the piece without playing it. If you have some time, it might be worth it to invest to learn some music theory so you can learn some basics about harmonic progressions and learn to identify them in the piece, and always have a basic sense of where things are moving.
Another thing I found helpful in really committing a piece to memory is to sit anywhere BUT a piano and imagine myself playing the piece (you can move your hands and fingers; feet if you need pedal). You may realize you need the physical sensation of the keyboard more than you realize, again pointing to muscle memory, which is unreliable in the long run.