wa alaykum us-Salaam
first, you need to cut contact off with him as he is your non-mahram....
i was going to mention that a sin is a sin and that if he drunk alcohol and gave it up, would you still go for it. if he didn't pray but gave that up, would you still so for it... so if he had complete changed, and was regretful, etc then i would have advised you go for it...
but you mentioned this as well then i think it is important for it to be addressed.
In my heart, I still love him and it's something that with time I think I may be able to look past it, however in my head I fear that maybe he's not the best thing for me because I am always going to feel like I'm being compared to someone else.
i dont know, its really up to you. its hard becuase you have to think about the affect it'll have on your relationship...what i mean is that these thoughts and feelings you have about this issue, could posien your marriage, and in all honesty i adive you NOT to ignore them. dont underestimate them and think they wont bother you later on. no! later on they may botehr you more as you would be married to him and expect better of him :)....
you know, you may always wonder that about him and your jealously and ill thoughts may affect you....who knows it may develop into suspicion...
i know it may be said that a big deal is only a bg deal when it is made a big deal-and it may very well be- but if it really bothers you that much, i'd sit and do some soul searching. write down why and why not he is suitable and see which would bother you and you think would negatively affect your marriage. need to put your love for him to the side and look at things practically. are you going to be able to control your thoughts, etc after marriage. if you dont, hoe could this affect your relationship/trust with your husband. prevention is better then cure
and dont for get istikhara :)