I was a serial cheater for years. My gf is emotionally abusive (has been verified by counselor) and to cope I manifested a sexual addiction. Now that doesnt make it right and I have reformed while still stuck with her. I found that cheating not only hurt her but was very unhealthy for me as well. Taking out the chance of contracting one of the many STIs it is hard to keep stories straight, the stress of being found out etc.. wears you down. Plus I found there really wasnt the connection there with random partners to fully enjoy the experience.
Have I tried to leave her? Yes. Does she know I'm no longer interested and want to leave? Yes. Does she accept this and want to separate civilly? No. She threatens, cries, begs, promises to change and uses sex. Now does that justify cheating? No. However it does make a poor decision seem easier to follow thru with.
For your well-being, you need to break the relationship and all future contact with her. You are enabling her sickness.
You are worth more, and don't need to be treated badly. Your worry should be the damage it is doing to YOU. You are not responsible for her behavior, no matter how sorry you feel for her. Every minute you're with her is robbing you of real joy in your life.
Depending on the home ownership/apartment lease, one of you needs to leave. If she is not on your mortgage or leasing contract, you can make her leave with a restraining order. If both names are on the housing arrangement, then you should just leave. FOR YOURSELF. You are not obligated to give her notice.