Why does she say this?

my ex and I broke up. when we did something she said really stuck out to me. she said "I hate imaging you with someone else, it hurts". yet she's the one that broke up with me. not me breaking up with her. so I'm wondering why did she say it or even break up? if she doesn't like thinking of me with someone else then why not stay? she described the feeling when she thinks of it as "its like getting kicked really hard in the chest". Is it a hint that she wants to stay but can't for some reason or what? help :/
Why does she say this?
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