Xynesthesia's post led me to think of this differently. It's possible that your struggles might be related to impulse control, at least in part. For things that are scary or emotionally taxing, the impulse to not do is stronger than the impulse to do, leading to a strong pull to not do.

A very experienced psychiatrist/therapist told me this years ago, that poor impulse control can lead to *not doing*. It didn't sink in then, but it sure does now. I wonder if you can relate?

If you've been trying for years without success, maybe think about with a new perspective--see if your therapist can help you with self discipline. Might be worth a try.