i am not a professional, but i believe that since your emotions are so strong you don't pay attention to the logical side of things.
it takes alot of time to train your brain to not get ahead of itself.
by what i have read, you do know that you overreact and that is a good sign
you have to learn to trust your bf and constantly tell yourself that the girls you see passing by probably don't know and/or aren't even interested in getting involved with someone else's guy.
also, being jealous will cause a huge strain in your relationship with your bf...if you really love him, you must trust that he only loves you. in his eyes you probably are the most beautiful person in the world

but, just in case something does happen (like if he cheats on you), take into consideration that it is also his fault. it would do you no good to be possesive of a bf who cheats.