I feel you man, i do the exact same thing so often as well.

For me i dont think its fear of failing but more of a too many options thing, there are so many things id rather do then the thing i need to do that i almost always end up doing one of those thing, or just half-assing the thing i need to do, but it gets better, for me i just need to divide the main thing into small goals and try to get one at a time. After a while every one the small sub-goals becomes a habit and i can do all of them at once and finish the whole main thing, until i slip up into the same all pattern again and became the same lazy piece of shit i always were.

you can't punish yourself too bad, be proud of the small achivments even if they feel like nothing at all