Jealousy, the passion killer...
If some one is making you jealous, it is due to their own insecurity. Reguardless if they are aware of it or not, subconscienly it is still ticking away. They will see it as a sign of weakness, a 'control symptom' if you will.
By making the other person aware that they are making you jealous, you can become aware of what their true intentions are. You have two options:
When telling the person they are making you jealous. Do this in a calm collected manner, make sure no alcohol is involved. Explain to them how you feel.
They may appologise. If you believe that this is a sincere appology, then they will try and rectify the issue. If it happens again, you know that something is up. Despite the fact that you have made them aware, they have not re-inforced your feelings.
Primarly, this shows that they are not really paying attention to your feelings, or even worse, there is actually something going on (eg infedelity, be it physical, or thought, either are, at the end of the day cheating).
You DO NOT NEED TO BE WITH THIS PERSON. It will drag you down, cause irreparable harm, which will fade but will not be forgotten, and indicate that the person is either leading you on, controling you, or worst still, having an affair.
It is only a matter of time.
Leave them. Why should another person have that effect on you. Ussually, if you love some one, jealousy drives a dagger of fear and lothing in to any relationship. This will in turn, develop in to hatred, hurt, insecurity, and worse of all, may cause you to do something out of charactor for you.
Then you will have the blaime set on you. This is not a good thing, you will never hear the end of it.
If the other person TRULLY loves you, they will let you go, they will respect your wishes. A person causing jealousy is trying to control you. They will argue with you, prevent you from being yourself, until the point where it looks like your fault.
They are not a bad person. They are mislead. They may have had bad experiences in life that is causing the issue now.
This doesnt make it exceptable behaviour.
If you love them, you will be doing yourself, and them, a favour. Leave them, you may at a later date you feel different, then it will be YOUR choice whether you get back with them or not. They will understand, trust in yourself. And if they dont, why be with them anyway?
Give it time, and thought towards yourself. You will heal.
Some cases may not include extreme jealous creating beehaviour, but all the same, it is YOUR life, it is YOUR decision whether or not you wish to be under any ones control or not.