I've talked to my T about such thoughts many times in the past. Sometimes he'd just check in, ask how frequent the thoughts were and how detailed. Other times we'd spend the whole session talking either about why I currently felt that way or as it's often the case for me that I have flashbacks to previous times where I felt like that, we talk about those.

I've always had good experience - regardless of what the exact topic is - with just saying very clearly what I needed and wanted to talk about. If it bothers me that my T doesn't go into enough detail with a topic, I'll tell him (usually beforehand, in your case I'd probably say something like 'I'd like to talk about some thoughts I'm having, but you always move on so quickly and I feel like you don't take it seriously and now I am unsure whether it's a good idea to talk about it'). Sometimes he has good explanations for why he does something and I agree, and sometimes he agrees that we can talk about it in a different way and we do that.