Jealousy is a nasty emotion, eating out the core of your being. It normally arises when you have the feeling that your lover pays attention to someone else, while you want it.
Jealousy can stem from your own uncertainty or be caused by a lover who is pushing your limits.
When you have a steady relationship you should talk about the behaviours that are and are not acceptable. Is it for instance okay to go out with someone of the opposite sex? How much time do you want to spend together and how much time do you spend alone or with your own friends?
Clear agreements, in which you set limits, tend to reduce jealousy.

What to do about jealousy caused by uncertainty
Appropriate jealousy

My own jealousy drives me crazy, while my girlfriend does nothing wrong. What to do?