Somethingnew, I'm sorry to hear you're feeling low.
You really risk tormenting yourself, though, when you still see this guy as he's dating someone else. Right now, of course you want him in your life still--he meant so much to you that you can't just turn your emotions off. But the "we'll still be friends" thing is probably tripping you up worse, at least while everything's still fresh.
Your brain is wired to see this guy and think, "Boyfriend." A cascade of happy-making neurotransmitters flood your system when you see him, but at the same time your conscious mind knows that that book is closed. The dissonance there just makes you feel worse than you would if you made a clean break of it. It is
bad juju to try to keep that friendship going while you're still actively hurting over the breakup.
You wonder how to let him go? It's tough.
Really tough at first. But you were able to let go of other things that light up your brain's pleasure centers, yet aren't healthy for you right now--if you weren't, you wouldn't have lost the weight you have. You have the strength to let go of him completely.
Sometimes it's hard to separate out how much you value him as a person and how much you value his former place in your life. Are you really missing
him? Or are you missing the things that he represented (a companion, a confidante, a lover, what have you) and maintaining contact with him in an attempt to fit him into that space that he no longer fits?
Either way, you're better off without him in your life for now. If you discover it's the relationship that you miss, then it's a perfect time to rekindle friendships that you may have let lapse or to form new ones. Re-train your brain to get those happy-making chemicals flowing for other reasons--going out with friends, doing something you love to do, trying a new hobby. Not having that relationship means having room for a lot of other things in your life. Prove to yourself that he is not your sole source of serotonin, dopamine, and oxytocin.
I hope you quit the habit that is him with few withdrawal symptoms. It really is tough, but it becomes easier each day that you manage it. Even Prometheus only had his liver eaten once a day; don't eat your heart out again and again by continuing to see the guy.