Wtf does “it’s not you it’s me” mean

I’m really struggling to understand the meaning behind ‘It’s not you, it’s me’ when my ex said that during the breakup. It feels like he was avoiding talking through what really went wrong and what I might have done. Even in the end, he couldn’t give me the space or himself the space to openly discuss where things went wrong or how we both contributed to the breakup.

It’s left me questioning everything. I feel like I’m stuck overanalyzing all the things I might have done wrong, but at the same time, I wonder if maybe it was him simply because he didn’t see a future with me, even though I saw one with him so clearly. Maybe that’s why it’s him—because he couldn’t or didn’t want to talk through things and just didn’t love me enough to try.

To make it more confusing, I recently found a letter he wrote to me last year, where he talked about how proud he was of our relationship and how excited he was for our future together. But now I’ve heard from friends that he didn’t see himself marrying me. I just don’t know how to reconcile these things—how could he say one thing and feel something else?

Has anyone else experienced this kind of confusion after a breakup? How do you stop yourself from overthinking everything when the breakup leaves you with so many unanswered questions? Why do people end things this way? It feels so disrespectful to our relationship and he even told me it sounded like a cop out…