People in monogamous relationships all over the world are very good at making it plain and clear that they are not available. This is done with certain carefully chosen nonverbal cues, such as not sitting too close, not touching, not smiling in a certain way, certainly carefully choosing words, and when it looks like it's necessary, just blurting that they are spoken for. This works remarkably well for millions of people. Maybe you can help teach her how to do some of this.

There is in life a thing such as setting oneself up for failure. Her going to nightclubs or parties without you looks like it might be one of those. Sorta like an addict not hanging out with other addicts who are still using after he/she has gotten clean.