Oh girl, you should just forget him!! He did it once, he'll probably do it again! You shouldn't have to be second best! There's another guy out there for you! Let this loser go so he can see how it feels to have a broken heart! And as it seems him and his ex deserve each other for both dating multiple people and stringing people along like that. Trust me when I say I know it's not easy! But I also know it will get better, and you can do better!!
Never regret something from your past, everything happens for a reason.