How do I get over retroactive jealousy?

Hi, thanks for reaching out!

It is really difficult sometimes in relationships to overlook a person's past, because as you said, jealousy creeps in, and also our own insecurities get in the way. There are a few ways you can approach your feelings in this situation that will hopefully help you focus on the present and not slip into thinking about his past all of the time. 

Communication is very important in this situation.  It is important that you feel safe expressing how you feel to him, and also that he is honest about his past relationships.  There was a reason those past relationships didn't work out, and he has moved on.  It is difficult, but you have to focus on putting your trust in the fact that he is here now with you in the present, and has moved on to that place with you instead of those people in the past. 

If you find that you are having insecurities about yourself, either from your present or from your past, it is important for you (and him) to recognize and focus on your strengths and what makes you a lovable, ideal partner in this relationship.  Build on what you like and enjoy about each other, which will hopefully help build a deeper connection where you don't have to second guess about his past.  Again, remember that he chose you and that there is a reason why.

Try to find ways to check yourself throughout the day if you find yourself overthinking or fixating on thinking about his past.  You can try some mindfulness strategies to distract you and regroup, or give yourself a check in, saying that you have control over your thoughts and you don't want to be going down that rabbit hole.  

If you continue to struggle with this despite trying these things, it would be a good option to talk with either an individual or marriage/family therapist to work on getting past these feelings and developing strategies to work through what you are feeling and experiencing.  

Best of luck, and feel free to reach out if you need anything in the future!