Anyone else struggle with rejection sensitivity dysphoria (RSD) and how do you manage it within your ethically non-monogamous (ENM) dynamic?

I've been seeing a guy who has a fairly chaotic & busy life (working full time, attending uni, has another partner, & then just other life stuff going on) ... He talks a bit about his other partner which I don't have a problem with at all. But there have been a few times when we have made vague plans (literally just that on this evening we will meet, but haven't pinned down exactly when or whatever) & he's then ended up cancelling either the day before or on the day because something else has come up. He doesn't always say what that other thing is (once he did say it was a work thing, for example) so I can't help suspecting that when he doesn't tell me, it's related to his other partner, whilst also being aware that is probably just paranoia on my part lol.

I've got some past issues that mean my RSD gets triggered when plans change like that. To give this guy credit, unlike in my past experiences, he is up front, communicates well, reassures me he does want to see me but just can't etc.... but this has happened a few times now. Unfortunately my go-to response to protect my feelings is to shut down a bit & I can feel that happening - emotionally I'm starting to withdraw as a protective mechanism. I have told him that I struggle with plans changing etc but due to his life being the way it is at the moment, there's just nothing he can do to prevent plans changing in the future too (not sure how long things will be like that for) & I'm worried my RSD will make me switch off & maybe even end things, or make him think I'm not interested, whereas other than this niggling thing, things between us could be so amazing I think...

So does anyone have any tips on how to handle RSD, both in terms of this specific scenario and in relation to ENM specifically?? Thanks!!