I don't even know what I "have" (cPTSD w/dissociative amnesia? DID? BPD? been dx'd with all at some point or another and I'm seeing someone new soon so we'll see) but I feel like it's important to make sure your therapist can recognize your dissociation symptoms. It might be useful for him or her to write down a summary of the session for you (that's what I'm going to ask when I restart therapy because treatment is a necessary trigger for me).
But yesterday for example I looked at the clock and it was 8:30PM and we were talking about world languages, and then all of a sudden I was confused and they were discussing God and Jesus and it was 9:20ish.