What kind(s) of open model relationship
one wants can change over time. At this time... Does all that mean you want a closed polyship limited to X partners? Versus being in the mostly closed "W" with the one partner also being "open?" What model does your newest partner seek?
It sounds like you are doing the right things once you realized it wasn't a one time thing but chronic triggering -- asked for partner support, set some initial boundaries to get a handle on it, started with a counselor, etc.
If you are going through a mental health/sttress/anxiety thing, I don't think it is unreasonable to ask a partner for some soft limits that could change in time as you get on to better footing and resolve personal issues. Could tell your partner what you are doing so he knows you are honestly working on it and not foot dragging -- he's already been to one appt with the counselor so it seems like he's aware you are taking this seriously. But could ask if he wants more updates along the way?
You both could redefine your boundaries later as you progress.
You don't sound great but sound ok enough under the circumstances. Just adjusting to change and doing things you need to do?
Could page 5 & 6 things
or this other article on jealousy
help somewhat with your interaction with your partner as you dig deeper with the counselor on your other stuff?