I think reading up about self sabotage can help you. It generally happens for one of several reasons including:

*success is scary because if you get good, you may have to work harder

*your self image is poor and succeeding at stuff would conflict with that which would feel uncomfortable

These are just two, but I think they are more likely than some others based on what you've written.

Good luck, and remember: the only time it's too late to take action is when you're dead.