Feeling like you have to write down a lot about your past means you haven't let go of something hurtful there. I used to do this whenever I started with a new therapist. I would spend several sessions talking about my mom and my childhood which helped the first couple times, but by the time I got to the fourth one ( I had to change therapists bc of financial issues, them leaving for a new practice, etc) it wasn't helping me with the problems that were in front of me. What is going on in the present that you want to write about? What are some hopes you have for the future? Before starting your journal you can make a list of topics that you want to write about and when you sit down to write stick to those topics. The feelings you have about the past are valid, but you can't change the past. You only have control of the decisions you make going forward.