I'm pregnant uncertain if I should keep it because I'm not sure if my boyfriend or fling is the father. Help?

I just found out I'm pregnant, however I'm not sure if my current boyfriend of five years in the father or a one night stand(I was really drunk that night celebrating my friend's birthday). The nest morning I took Plan B and I was told by the doctor that Plan B doesn't work if you're already pregnant. I had sex several times in January with my boyfriend and only once with the other guy. However, once I found I was pregnant I also discovered I was ovulating when I had the one night stand. What makes things worse is the fact that my boyfriend and have been having unprotected sex for three years and I never got pregnant. I really love my boyfriend and this was a terrible mistake, we've been discussing marriage lately and I'm so scared to tell him I'm pregnant and the baby may not be his. If I knew for sure the baby was his I'd keep it but I'm not sure. Should I just tell him I'm pregnant and not mention the one night stand? Or should I tell him I'm pregnant, mention the one night stand and the possibility of the child not being his? Please help! I really don't want to lose my boyfriend but I'd hate to have to abort my baby just to save this relationship. If I abort the baby I wouldn't mention anything to him at all, and this between us would remain the same.