I'd go with a Just Do It approach. Answer any questions he has truthfully and completely, and otherwise keep your mouth shut and do what you're planning to do. Words mean nothing at this point, and actions mean everything.
If he asks "how do I know you really mean it this time?", ii]then[/i] you can remind him that he doesn't really have anything to lose. If you go back to the way you were before, he can always leave you then... you're not going to get pregnant in the meantime, right?
If you run into trouble keeping up your end of the bargain, go talk to a professional without delay. Don't let yourself fail again, and if you can see yourself failing, and you don't know how to avert it, realize that you're not just going to invent a way to better yourself this time out of the blue any more than you did the last couple of times, and get whatever help is called for.
One thing you may not have tried before: exercise. It does wonders for your attitude, your libido, and your confidence. You feel sexy and you are sexy, and you can't keep your hands off of each other. Go to the gym, get a trainer to keep you on track if you can possibly afford it, and rev up your body so it can send more and fresher blood to your brain and keep it firing on all cylinders.
a fine and enviable madness, this delusion that all questions have answers, and nothing is beyond the reach of a strong left arm.