How do you go back to someone after time?

My ex and I were completely in love. We were the relationship I'd always wanted. We dated for a long time and we were so happy! We talked about getting married and having children and having a life together. But then out of nowhere he began using drugs and it turned him into Dr. Jekyll and Mr. Hyde. We broke up and haven't dated in 6 months and haven't talked in 4. We just started talking again, and both of us want to try again (he got off the drugs and quit using. I don't want anyone saying it can't be done etc etc that's not what this question is about) but I am extremely jealous and have high anxiety. We have both been with other people, and his life and friends are nothing like mine now. How do you start the relationship anew without being upset about what happened during the time you were broken up? I completely understand that he had every right just as I did, but how do you get over your ex being with someone else and how do you get over the fact that they became something awful for a while? How do I accept him for who he is and who he was?