Quicksand is a really good description of the experience. If you can't catch yourself at the start of the sinking feeling, maybe you could catch something further down the line and say something intuitive like "This feels like quicksand". I am speaking to myself as well, being able to say that would help me. The important thing is to say something which draws attention to what is happening rather than behaving from habit or without curiousity. Our therapists can't always articulate that because they don't have access to our internal worlds, but with practice and some hard work we can go some way towards doing that for ourselves.