I just want to chime in and say that I don't
assume your issue has anything to do with somehow mentally not being on board with poly.
When I was single and dating a few years ago, I started having sex with a guy who seemed into me but similarly couldn't seem to maintain an erection. At first we thought it might have to do with not being used to condoms, but eventually we stopped using them, as neither of us was seeing anyone else, and it still didn't fix the problem. He too had recently been in a long term monogamous relationship (8 years).
Anyway, one thing I noticed was that in the morning, I could wake him up with a blowjob or something and maybe the unexpectedness of just getting down to it like that defeated whatever anxiety was going on because... it worked out in the AM
So maybe work on ways to avert the anxiety circuit by having sex in more *unexpected* ways?