Jealousy is fragile weapon. Men will be jealous on your sex partners, women will be jealous on romantic feeling toward other woman.
That is an old truth. If guy writes you romantic letter your partner will smile, but if he wants to have sex with you, your partner is ready for fight. And that is normal, every male protect his ego and his right. Even romantic letter is not so harmless if this is background for something more, or trap for sex. So, when man can loose his partner, he will fight and kick ass of other man.
What to do when other woman wants to steal your partner?
Women are more complicated. In this race, woman will even pretend that they like their rivals. They will make friendship, give compliments and then make sabotage to their competition. Woman will prove why she is the right one, and why is other woman the wrong one.
Woman doesn't fight with fist. Woman fight with sex appeal, high heels, short skirts, smell of perfume. Because, men are weak on sex appeal, and when such woman is also kind, gentle woman, she has big chance to win.
So on one side we have jealous woman who will complain, who will be desperate and on the edge of nerves because other woman wants to steal her partner. On other side is coldblooded rival in high heels. She can be secretary, neighbor, maybe even his boss. But when that cold monster decide to steal your man, watch out and don't fight with fist.
You need to test is your man loves you as you think he do. Focus on your man, does he miss something in your relation. Make competition invisible and unnecessary. If he talks about her, tell him "i don't know who is she".
You have power to talk about subjects which involve you and him together. For example :"Remember that we watched Hunger games, now is part 2 in the cinema, i have tickets". Find things which connect you, but give him space to use his free will.
Once when when you get him, don't pressure him, don't let man choke in relation with you. Don't force him to forget his friends and family, accept his baggage from the past and show him the future with you.
All is allowed in love and war, but we must be careful in the choice of weapon.