How can I overcome feeling jealous about my significant other's past relationship?

Hello, it is very nice to meet you. You did the right thing by reaching out and asking this question. If you and I were working together in therapy, I would want to know more about your current relationship. You mention that this is your first relationship, but that your partner has been in previous relationships. One very important thing to always remember is that they are with you now. They chose to be with you! Most everyone has a past, had been with someone or done something, somewhere along their life path that they may later regret. That is how we learn life lessons. 

You are in control over how you react and respond to your partner when you are together now. Remind yourself that it does not matter what they did with their ex - it is in the past. The problem with continuing to bring it up is that it poisons the present and probably causes stress and tension when you two do spend time together. Do not give the ex the power to ruin your time with your partner now. Think of the things in the relationship that make you feel good and feel happy. Remind yourself of those things if you find yourself feeling jealous. Don't allow the past to steal your joy and happiness in the present. Another thing to consider is - what does knowing this additional information do for you now, in the present? Why does it matter? 

Think of the past as the past. We should have no judgement over anything someone did before they met you. Now, if your partner is still in contact with their ex, that would be a completely different issue. You would definitely have a right to be concerned and unhappy with that.

Remind yourself that everything in life falls into one of two categories, things that we have control over (ourselves, our actions, our words, etc) and everything else. We have no control over the past or what others around have done.

I thank you for taking the time to write this question and I hope that you have found it helpful. I wish you all the best moving forward in your journey.