Correct me if I am wrong but...
I think it's because you have learnt the pieces to quickly and didnt practise them properly while learning them. You didn't spend enough time on them to truly know them well.
By practising properly I mean, taking every section, phrase by phrase, bar by bar, and not just skimming over it until it is sort of right, but working every single detail into your head and fingers. I find that the more you practise a section and critically analyse it, until you can practically write out the notes from memory, the less chance you have of forgetting it while in a performing situation. Constant repetition. Regardless of how boring this may sound, it is effective. Work out the boredom, develope your own little techniques and tricks while practising. Ie, keep your mind occupied then you will not get bored.
There sounds like there is more to the problem then you have posted so if I can help anymore, just ask.