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If you feel that continuing to see this person is going to hurt you and that she will put her interests first, then it is probably a good idea to end all contact. While some people can be friends with an ex, emotionally it is very difficult and because of that it is often a good idea to move on. Also, if she has affairs, frequently changes relationships and is only with her current boyfriend because of sex, those are often signs of someone who has difficulty caring about others and forming solid relationships. So even as a friend she might not be that devoted to you. It's better to find people you can bond with who share loyalty and true friendship with you.
If you find that you still want to be with her even though she is hurting you, it might help to talk to a therapist to find out why you are attracted to someone who is not nice to you. Sometimes self esteem can be an issue in relationships. Therapy helps a lot to uncover why and how you can deal with it so you are happy in your relationships.
I hope this has helped you,
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My best to you,