I would guess that your definition of "successful" is set unrealistically high, and/or you've burned-out when it comes to moving above what you consider to be "mediocre". The former can also lead to the latter.
I recommend re-evaluating what will actually make you happy. Take a critical eye to the expectations you place on yourself and make sure you're only hanging on to the ones that will make you yourself happy, not the ones that society, or your parents, or anyone else has given you. Alternately, for some people, having expectations at all is problematic. Some people (like me) thrive in the absence of pressure and get paralyzed by it when it is present. It could be that you just need to give yourself a break and ask yourself what you want from life, instead of what you think you're supposed to want. Everyone's different.
Best of luck working through all this.