How to break up with a psycho girlfriend?

So me and my girlfriend been together for 6yrs now. I've tried to be very patient but her controlling possessive and tends to act like my mother behavior is too much. I've never cheated not one time.. 2 or 3yrs ago while she was locked up I almost broke up with her and started dating a mutual friend but didn't bc she said she would change it and she still hasn't. I use to be the same way but after evaluating myself I made a big effort to change and have. What should I do?
How to break up with a psycho girlfriend?
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