Why it lands as a verdict
When someone leaves, the mind compresses a complicated situation into a simple sentence: I was not enough. The compression is efficient and wrong. It discards timing, their circumstances, what they were capable of, and what you had already stopped getting.
It also generalises. One person's decision becomes a statement about all future people, which is a leap no evidence supports. Noticing the leap does not dissolve the feeling, but it does stop you treating it as information.
Why it physically hurts
Research on social rejection has repeatedly found overlap between the neural systems involved in social and physical pain. Whatever the precise interpretation, the practical point stands: this is not weakness or drama. The body is running a genuine alarm.
That also explains why reasoning with it does not work quickly. You can know the argument is unsound and still feel the full weight of it, in the same way knowing a burn will heal does not stop it hurting.
The comparison trap
Looking at whoever came after is almost universal and almost always makes things worse. You compare your interior — three months of poor sleep — with their exterior, which is selected and posed. The comparison is rigged before it starts.
It also answers nothing. Even if you knew every detail about them, it would tell you nothing about why the relationship ended, because relationships end over compatibility, timing and effort, not rankings.
What actually shifts it
Not argument — evidence. Being wanted in small ordinary ways: a friend who calls, work that goes well, a stranger who is pleased to see you. These accumulate slowly and do what reasoning cannot, because the belief was never built from reasoning either.
If the feeling predates this relationship and has followed you through several, it is worth examining separately. A belief that recurs with different people is usually about something older, and it tends to respond well to actual work on it.