Everyone has a past. Whether you like it or not, you also have a past. It might be a past relationship, a bad habit you already got over with, a flaw you had corrected, or the likes. I know, your past makes you the person you are today, but you don’t have to revisit it from time to time.
Do not dwell on your partner’s past, instead focus on the present and plan for your future. There really are things better left unsaid.