Why am I so jealous of my boyfriend. Constantly think he wants to or is seeing someone else

You are feeling the jealousy due to low self esteem on your part. If you liked yourself better you would not even worry about him cheating/seeing other people.

Yeah... you should give your esteem a boost.
Jealousy can push your loved ones away.

If there are no signs to worry about him being with someone else, then relax and enjoy your love for him.

Have you ever been cheating on by one of your boyfriends in the past because this might be why your feeling this way. If so know that everyone is different.

Now you realize your issues and now you need to work on it. Having insecurities can be a deal breaker for a relationship and can only push that person away whether than pull them closer.

Low self esteem is one of the leading causes of relationship problems. You don't feel good about yourself, therefore, you feel he will find someone better . Get into some counceling to find the things that are great about you and what makes you happy... you will feel better and won't be so worried