Ok my friend, here's my answer:
Since you are a man, let us analyze this as the logical human beings that we are..
- She says she loves you
- She has sex with another guy
- She want to be with the other guy, but not with you
- She brings her defenses up every time you try and talk her out of it
- She threatens to kill herself when you try to leave
So, here's all this analyzed pretty accurately:
She does not love you. She is using you as a safety balnket and I will explain to you why.
This other guy that is getting the best out of her, is what men call a PLAYER. He knows female psychology so well, he knows exactly what makes her tick. This girl has a lot of self-esteem issues and want the best of both worlds, because one guy gives her sexual pleasure and another looks after her (women basically need both of those things, SEX and SAFETY).
OBVIOUSLY, you cannot talk with her about it in a negative way, because women do not respond well to logic my friend. That's us males.
Anything you say or do, she will interpret as you being selfish and egotistical. The reason she thinks like this is because, she is the manipulative, selfish and egotistical low self-esteem person in the picture. But, women are very good at manipulating and getting what they want, especically when a guy likes them. They use sex as a weapon and their body too.
Think of it this way, if she really wants to be with the other guy. As soon as you leave, guess what...SHES GONNA BE WITH THE OTHER GUY. she's not gonna commit suicide or attempt to hurt herself, because that's only a cry for attention. Women love attention, and love to seek approval, well at least the low self-esteem women.
The only way you can look at this, is the healthy way.
The healthy way is to think of yourself and ur needs FIRST.
If she is too much for you to handle, then move out, go away.
She is fine, and will learn for herself that she is making a mistake.
This other man will eventually get bored of her and dump her, so he can get other women.
Do not try to logically persuade her, she will not respond.
The only way you can turn this around, is a dangerous and possibly destructive way.
Which is, you become a man and get what you want, which is her. You show her you are a TRUE MAN, and sleep with her. This is only destructive because it may backfire, and she may start sleeping with both of u, and it may destroy u, because you don't seem the type of guy who can ignore that fact, mainly because you probably don't have enough confidence in the person you are, to know that in the end she'll dump him and stay with u.
So, the alternative is, put ur foot down, and send her packing OR move out.
Do not feel guilty for doing so, its her life, she does what she wants with it, you cannot control her you can only control yourself!
She will only get worse, you cannot fix her.
She is not ur responsability, and as a MAN, ur responsability is URSELF.