First off, please know that love doesn't act like this. Even with substance problems, cheating is more about his character. Lack of respect or regard for your feelings is showing there.
Please don't accept so little from a relationship, unless you only want to give and never receive from it. Falling love instantly is a big red flag, really. Real love is based on how we feel about the real person and all their qualities.
You can be enamored with someone and addicted to the excitement, but true deep love is something else. It would be worth your while, I am betting, to find someone who can return your affections without hurting you.
don't accept so little.. life is too short.
Been there...done that...but no more. I am worth more than that. so are you.
One more thing... he does not need anyone else to help him work through this. Millions of folks do it on their own, with no support. I sometimes think that may be more of a motivation than anything , to realize that no one is going to be trying to fix you anymore.
When you are all you've got, thats when you find your inner strength.
hugs
chic