How do I get unattached?

Can you tell him that you believe there are issues between you that getting back together so soon isn’t going to fix, and that you need some no contact space to figure out how you feel?

I’m sure you broke up for a reason, and the fact someone chose to break up at all doesn’t bode well. That’s supposed to be a last resort when there’s no solutions to relationship conflicts. And while there are some rare situations where it’s a mistake and should be patched up, it usually doesn’t work to break up because you couldn’t work out the problems and then decide it’s time to try to work out the problems after the break up instead of before. It creates a lot of instability and distrust to break up, on top of whatever problems there already were.

It makes a lot of sense you have doubts, but see what happens if you pay attention to your intuition and instead of your fear. Take some distance to clear that up for yourself.