I think you nailed your trigger.
The anxiety issue maybe linked to my abuse/rape by an ex, he would cheat on me, blame me for it, and I'd have to have sex with him to make everything ok again. I'm working through that with a counselor now, my abuse and rape I mean. It could be related and linked in my mind but I'm not sure thats what is causing the anxiety.
Could just be up front with BOTH of them.
She may not be your GF, but she is your metamour.
Until your time schedules change again, could propose a "for the time being" solution --
Could ask them both if they are willing to go slow.
Could ask John if he is willing to reassure you that he's not going to be making out/having sex with her and then unfairly blame you for probs in the (J+M) tier of the polymath ask you to service him to "make it ok again" like the abusive ex did.
Could ask M to be aware you are in therapy for past BF abuse/rape (don't have to TMI) and if she is willing to go easy. You want for her to enjoy being with John but if you get an anxiety attack, not to blame herself or (blame) think you hate her and just come to you direct for more info. Or if there are problems on the (J+M) tier of things not to be unfairly blaming you for them.
Lay it out there, ask for their willingness to try it on for the next (however many weeks) it is til work schedules change again and you all have to reevaluate the dating schedule again and deal with time management needs.
You may find being assertive and communicating clearly about it helps soothe the anxiety/emotional flooding/panic attack down. And DO take your meds as directed. It's part of your self care just like working with your therapist is.
Hang in there!