I am not sure that motivating yourself is truly the problem. The problem is that you do not know what you need to do specifically in order to be challenged while practicing, and while performing. I think (I am playing mind reader here) that you are limiting your performance to “wrong notes/right notes”.. am I correct? When there are millions of things you can propose musically and emotionally speaking that would make you (and therefore us) not interested that much about “Did I play the right or the wrong notes”? Now, I am not saying that playing super sloppy is a goal!
Try to think of it this way: You speak a language correct?.. So when you listen to someone saying something to you, do you care much if him/her is bumping on some words? Or even saying one word for another? Would you then think that he/she can’t speak this language? Do you think that a computer who reads a text (regardless of what this text says btw) is “interesting” in any way? Do you feel emotional when a computer says something?
So what's the goal of playing/performing? It’s to project emotions… But these emotions you have to have them first in you, and then you SHARE them with the audience, because you want the audience to hear what YOU think is beautiful, what YOU think is worth playing in this piece, what makes YOU cry, happy, in this piece etc.. You get my point?
So maybe let’s start by going back to your eventual lack of motivation at the piano, and figure out WHY we are pushing on those keys… We are NOT pushing those keys to please your parents, friends, relationships etc.. We are pushing on those keys because we LOVE this piece and we want to SHARE that love around us.. as if we are talking to our friends, family, and let them know about this wonderful place, this magic location we recently found, this amazing movie we just watched, this fantastic book we are so excited about, this gorgeous picture we want them to see etc.. and please let me ask you this: when you are sharing that love you have for something (or someone), do you feel you need to be "motivated" and because you may not, then you chose not to talk and/or show anything to anyone? ever?
I think I made my point, and I hope it will help you all to understand where the motivation should come from... from YOUR love of what you are doing and YOUR will to share it because you are excited about it!
Please let me know your thoughts, I always enjoy feedbacks.