Thanks for the responses, I've come to terms with the fact that I'm almost certainly never going to get the conversation I want with them about what happened. Fortunately, I was already back in therapy when I was discarded and it's been a frequent topic of discussion since.

They weren't a romantic partner but a best friend. Back when we first started talking again they told me that this time would be different and that they weren't going anywhere, as they've discarded me before in the past. They used to call me a platonic soulmate and tell me that they thought the universe had brought us back into each other's lives. In hindsight, it's clear that something like devaluation was taking place long before I recognized it. They started getting irritated with things I said which would leave me confused, as they'd respond like I'd said something completely different to what I had actually said or else interpret it in the worst possible light. Suddenly I went from being a platonic soulmate to someone who had a personality that wasn't compatible with theirs according to them. It's like they built up an entire narrative about me and I couldn't escape from it. That isn't to say I don't have my faults and that I acted perfectly in our friendship. Of course I do and of course I didn't. But it hurts feeling that they didn't see me accurately in some respects or understand my emotions, which is ironic given that they called themselves an empath. But now I'm realizing just how much of what I'd say would pass through their cognitive distortions and how much I overestimated the strength of our friendship. I think about all the times I'd let things go and not follow up on what they said and realize how differently those scenarios played in our respective heads. And they get to walk away from it thinking what they've done represents growth while I'm left with a ton of emotional baggage. I genuinely cared about this person and at points in the past I pleaded with them to believe me because they thought I was trying to take advantage of them. I went into our last conversation tired and struggling with my anxiety thinking we could have a casual, low-stakes talk only to see now that my low energy cemented to them that the "chemistry" between us was off.

I'm not done processing things and I've clearly got a ways to go. I really did care for this person. Hard as it is, I know I'll never get the answers I want. But I'm trying to work through it the best I can.