Jealousy (Need Advice)

Ok let me start this by saying I'm the most jealous person on the planet! I try to work past it or ignore it but it just gets to me! My entire life I've seen the women in my family beat up other women or even their men over it (horrible I know) but it is what I've known my whole life.

So that's why when I see my fiancé talking to another woman I just get so angry at him!!

The few and short lived relationships I've had in the past have been with really shy guys so there wasn't much competition but my fiancé is pretty charismatic and it seems like girls really like him -___-

We have fought about it before where he says I'm ridiculous and he loves me and he wouldn't be with me if I wasn't the one person he wants to be with. But I honestly can't help it, it's an automatic reaction.

Maybe I've been spoiled by stories or romance novels but part of me feels like I should be the only nonfamilial woman I'm his life. I still talk to my guy friends on fb and Instagram that's fine if he wants to do that with his female friends. But I kind of draw the line of hanging out in person alone! Am I being ridiculous?

He doesn't have a laptop so he has been using one of my two and my new one died so I was on the one I gave him and i logged into his Facebook to post some funny embarrassing hacked statuses and I know it's horrible but I snooped in his messages :( I know it's wrong but i couldn't resist the little devil voice in my head telling me to do it. Anyways I found a message from his really close female friend that I've always kind of hate for the sole reason that they're friends and my fiancé told me before we left California that she asked him to go hiking with her. Well in he messages I found that it was his idea not hers. Is that a big deal? Did I hear him wrong to begin with and I just automatically blamed his friend? I don't know -_- anyways I saw another female friend (named Hannah) in his messages nothing bad but when he came back in from playing horse at the basketball courts down the street last night and asked if I wanted him to make me dinner I said "no but call Hannah I'm sure she'd like some"

I'm such an asshole sometimes -_- I know it but ughhhh I'm like ten thousand percent loyal to him! Why can't he do the same for me?

I know I'm being super immature!!
My question to you guys is how do I overcome this? Help!?