It takes time to get to the level where you can command your emotions like that. Have you tried reading the pavlina article and doing a google search?
overcoming jealousy - Google Search Overcoming Jealousy
Let's try this another way: say your suspicions are correct and your bf is cheating on you. What would happen? Would you be able to handle it, or at least, survive the process? If so, then why worry ahead of time? you can handle it. If you cannot, then grow and learn to trust yourself enough to know that you can handle it. "Feel the fear...and do it anyway" may help you in this regard.
You've identified another issue there yourself (though I dunno about this) of self esteem, have you done anything about this? You can try therapy, or simply a counsellor who's willing to listen. Or you can read books like The Six Pillars of Self-Esteem.
Journalling my help, ask yourself these questions: What would happen if my bf cheated and I found out? How would I feel? How would I react? Why do I feel posessive of my bf? Why do I not trust my bf? Is it because I may not trust myself in some way?
Good luck! You'll be fine. Acknowledging the issue is the first step to solving it.