Why do some people not help their partners get better mentally when they did it for them?

I spent 4 years driving my crazy helping my ex girlfriend get over her mentally health issues after something bad happened to her. I ended up going though some depression of my own most cause from things she did and other personsl issues and she left instead of helping me. Now she's acting like i was a bad boyfriend. Why do people do this?
Why do some people not help their partners get better mentally when they did it for them?
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