Don't think of doing the whole thing. Just "investigate" it and let yourself spend 5 minutes making a list of what still needs to be done (and then walk away). Break it down into smaller tasks that don't look like the whole project and quit using your energy to tell yourself what a failure you are and how you can't do it. That doesn't help at all so make yourself stop it. Focus on what you can do and you'll go in that direction.