My partner’s drug past triggers me for unknown reasons and my past traumas. I just don’t know why and how to get past this?

I love near everything about them

This is the past. I know it’s the past. I just can’t process that their now isn’t necessarily their past. They tried and decided to change. I just can’t get it in my head. It triggers me. Maybe my past trauma around it. I don’t know. I just want to get over it and don’t know how. I am talking to a therapist.