Your too much of a nice guy!
This girl was sitting there...demanding that you two get married someday soon, threatening to kill herself, and threatening you in other ways if she didn't get her way... Funny thing is that now that you ended it, she's begging you for another chance.... Yet she was the one ALWAYS threatening to break up with you if she didn't get her way.... Don't you see something wrong with this picture?

Anyways, Your are being WAY too nice to her. To even feel bad about breaking up with her....and wondering if you were too harsh..etc. No, you just put your foot down for once in your life..! Stick to your guns! If you give her another chance....chances are shes gonna do the same....and maybe even worse. Then by that time, it will be harder for you to end things with her....

Make a clean break now and just tell her to leave you alone and then YOU need to move on! Don't feel bad...she sounds like a psycho.

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Appreciate the good times and learn from the bad times....