You can decide to be better at something. Let me explain.
Obviously you cannot make a decision like this and then immediately be better, but what you can do is make a decision and then figure out what to do to make it happen.
A little while ago I decided to be better at piano playing. This involved, first of all, abandoning the belief that I could never be very good. That belief led me not to practice technical things like scales, or not to practice, because I thought it would be pointless: I would never have good technique or time, because... Well, because I haven't achieved those things yet.
So now my practice is focussed on working on specific things that need to improve. So the decision to be better did produce almost instantaneous results, in that one day I practiced one thing for a pre-determined length of time and that one thing did in fact improve. I am now better, and every time I make a decision to be better, I am. It is a daily decision I make now.
There is nothing unrealistic about this. I am not trying to be better than anyone else, but better than myself, so I don't have to worry about any other piano players out there.