I had an LTR previously. What I have noticed so far is that as soon as I got a little bit jealous towards one particular guy, she suddenly went into full-interest mode to him, eventually breaking up with me. I am sure there were some other things that lead her to make that decision, whatever.

How do I act if I become jealous? I understand that it does come from insecurity, but, still. Should I try to befriend the guy, showing this way that he's no way a threat? What if she's jealous towards me? How do I act in this case? Reassure?