You deserve someone who's as "all in" as you are. Therefore, you need to have an honest, open conversation in which you tell her calmly that you know she makes sexual advances to other guys. Describe how it makes you feel -- embarrassed, angry, hurt, betrayed, sad, etc. Maybe she can shed light on why she does it. For example: maybe she's not ready for a committed relationship, she feels insecure about herself, you feel more strongly about her than she does about you, she doesn't perceive the gestures she's making to be sexual advances at all, etc. This will be a difficult but worthy conversation if you don't allow emotions to hijack it.
For your own dignity, you have to decide if you're okay with her behavior. (Obviously you're not. You've written me!) Why make this decision? Because currently, you're giving her your CONSENT to treat you this way every time you take her back. If you're not okay with it, explicitly tell her so. Redraw your boundaries, meaning establish what a relationship dealbreaker is. Then STICK to those boundaries. She may realize that she lost a loyal and loving partner a little too late.
I wish you all the best in standing up for yourself.