If you are truly caught in a place where you're not sure who you should be with, quite honestly you should probably not be with either of them. If you're with someone and considering leaving for another person, then you aren't 100% committed to the person you are with. You should not leave to be with someone based on infatuation, attraction, etc entirely either. You know the previous partner was left for reasons other than feelings, right? So think about this, if it was what you call "love" in the first place why did you leave? By that logic, then why would you go back based on that alone, knowing what you do about your ex?

Relationships are motivated to start by feelings of attraction, infatuation and such but that is not what relationships are built on. True love is not something you feel in your heart but the everyday choice to remain committed and in a relationship in spite of feelings that may come and go over time.

take time to think about this and if you cannot forsake the old lover then leave the one you have and don't get in a relationship until you can be 100% committed.