I suffer from big rumination problems. It's not as bad with regard to the therapy as some other things, but still, the following helps me.

If something is bothering me about a session, I'll jot down quick notes. I can then review my notes before the next session and see if it's still bothering me, and if I want to bring it up. It often helps me put it aside in the mean time. (I guess in part because that's what therapy is for, and they have to put up with it. So I know I can try to get it sorted at that time -- decision made, that's done. Find something else to fret about. )

I have a small, pocket notepad I use for therapy notes and ideas. I don't like to waste time in limited/expensive sessions or potentially forget things and then kick myself because it's another two weeks before I get another chance to deal with it.

When I go into a session, I can review any notes and decide on priorities and if I even want to deal with certain things at all at that time. And knowing that it won't slip between the cracks, because I've made note of it helps me not stress about it.