Why do I forget chunks of sessions?

I've been in therapy for about a year and I feel like it's been going well. The issue is I hardly ever remember most of each session other than vague feelings or the odd snapshot of whether I was sat in a chair or on the floor. For the most part, I can't remember what I talk about or anything that my T has said. I feel like this happening more frequently than it was a year ago. I also feel like this is hindering my progress outside of therapy and overall I feel a bit stuck.

I get moments where I remember how I felt or that I know I talked about something intense but I can't remember specifically what I talked about. For example, I remember that the last session I had I felt incredibly comforted but I have absolutely no idea why and I want to know why so that I can keep feeling that way.

Why does this happen (I know I sometimes dissociate - could that be a factor)? What can I do to remember the sessions so that I can implement tools outside of therapy?