Why would you want to be with someone who doesn't want to be with you? You need to take a good look at yourself. I don't know that you're so much afraid of not having him as you are afraid of being alone.
Believe it or not, I've been where you are. I've had guys leave me because "someone else" has come into their lives. It doesn't feel good at all, and it downright hurts to be second-best. But I've learned that these guys don't deserve the girl they've hurt, because if they could see how amazing they really are, they wouldn't have left in the first place.
Take some time for yourself. Really get to rediscover what makes you happy. For me, that was music. I spent two years single after a horrible break-up like yours. That whole time, I focused on myself. I decided what I wanted to do with my life, and I set out to do it. I'm dating again now, and this guy really supports my goals and wants to be with me for me.
You need to find someone who'll love you for all of you, but first, you need to learn to love yourself. You're so much better than what you're describing now.