It takes time, but you will do this - you'll achieve your goal.
I don't believe having a younger girlfriend is going to work out that well for him. It could be that it is a great love story (in which case, be happy for him if you love him - everyone deserves that!) but relationships are difficult. That relationship may fizzle.
By the time you've lost the weight you want to lose, taking the amount of time that you need to do that, you may not care anymore how hurtful his remarks were. You'll be too busy with other things in your life and maintaining a healthy lifestyle. His negative influence in your life will feel unwanted and out of place. I hope so.
When he is out of that relationship and reappraising you, aren't you going to wonder why he didn't love you for you, not your measurements? Apply his own standards to him: if he lost his hair, or gained a lot of weight, would you dump him?