Hard time with partner breaking up with meta

I (f/36) and my new partner of 3 months (m/43) have been fully in NRE and enjoying our connection (we run in the same social circles and were previously acquaintances). However, now he is in the process of breaking up with one of his other partners. This is putting me in a situation that feels awkward. Their issues predated our relationship but I'm very aware of the fact that he sees me as more compatible in some fundamental ways and I have seen him unconsciously and consciously comparing the two of us. So this is making me feel a bit like the other woman. I've never navigated a meta breakup situation before. Like, if this was a monogamous situation, I would probably avoid being with someone fresh of a breakup. But in polyamory, I don't know how to respond. How do I deal with him, presumably, needing some time to grieve that relationship when we are so new? Also, I have fears that I'm the next one to be replaced by the shiny new thing.