I completely trust my boyfriend, probably more than I've trust any previous boyfriend. He is my best friend and my family. We moved in together a few months ago, and I realize how jealous I feel inside. I feel distraught when he mentions his past. Even if he doesn't mention his ex girlfriend in a story during a period where he was with his ex, I imagine her there. I even imagine him and her laying in bed and cuddling the same way we do now, and it hurts a lot.

I also don't really like it when he speaks to other women, unless they are in a long-term relationship. I know it's my problem and I'm wrong, but I genuinely feel enraged by even knowing he would talk to another female or think any girl besides me could have enough value to him to even be considered a legitimate friend.

I love my boyfriend more than anything but sometimes I think about leaving him due to my constant violent mixture of emotions - love and heartbreak - the sort of pain I imagine I would get if he left. I just don't want to hurt anymore. I know he loves me and he has never, ever given me a reason to be suspicious of him.

We went to high school together but weren't really friends but five years later life brought us together 1,100 miles from the town we grew up in. In high school he was sort of a player but he was young and immature and he is infinitely a different, better man now. Since we've been together he's never hidden anything from me, been completely open about everything, and not lied.

I never snoop but he leaves his Facebook open on my computer or has me answer his phone or check his texts or whatever and that does comfort me as it lets me know he has nothing to hide. Nevertheless, the thought of his past girlfriends laying with him and doing private things with him haunts me, as well as the immature, cheating high school boy I used to know him as (even though I'm aware that's he is different now---we never dated in high school but we went to a small school and everyone knew everyone else's business).

Basically I am just having these obsessive thoughts and they are ruining me and making me lose my mind. Please help with advice and responses.

This is past normal, and you should consider professional counseling, to get over obsessive thoughts, We all have pasts, he could have several lovers, in the past, and yes they cuddled, and he did fairly well the same thing with them, as he does with you. That is life and how it works, he left them, and you have to accept these facts.

How old are you - about 24? You were a virgin when you met him, after all those boyfriends you mention?
I agree with above - this is neurotic to the point of needing therapy, especially if you have a sexual past too.
I have a standard speech I give about curbing jealousy because if you don't, you lose him. We all get jealous (about looking and flirting more than the past loves), but we put a lid on it, and for healthy people it can be handled. But I have a feeling you aren't going to be able to 'see' the obvious.
As for why you do it, we don't know your life. This kind of irrational jealousy is selfish to the extreme, like a spoiled child who doesn't have to share her toys. Perhaps you got everything you ever wanted from your parents. Or the opposite - perhaps you developed no self-esteem because you weren't loved. Or a mix of the two - working parents who gave you everything you wanted, but without love.
Sorry to be harsh, but if you don't do something soon you are going to lose this great guy.
If you want to talk here about why you think why, feel free!

I am 21 and he is 22. Yes I have had past boyfriends and was not a virgin when we got together, but I would definitely not consider any of my past relationships serious. This is my first serious relationship, but not his. He has lived with a girlfriend before, he has been in love with two other girls in his time. And I think that's what I have trouble reconciling about even more than past physical intimacy; it's very hard for to grasp the concept that he can love me even though he once loved other girls that way and it didn't work out.

That's probably because he is the first and only person I've ever been in love with or been able to open myself up to in this way. I consider what we have sacred, but then I think about him and some past girlfriend and I imagine them being in this kind of intense amazing love and I don't understand how he can be over that (I don't believe I will get over this ever). I will marry this boy I love him with all my heart and, in a way I didn't think was possible, finding him really was finding my other half. He is the one person I've found in my lifetime (of a romantic relationship or otherwise) that I can be myself completely with, tell all my little secrets and thoughts to, feel safe in his arms, talk forever about things or even just be quiet together.

And then my crazy brain starts drawing comparisons between me and past girlfriends, which I know is the danger zone but however can't seem to stop the intrusive and unwanted crazy thoughts (for example, does he love me more now then he loved his ex-girlfriend then?)

This feels horrible and I am trying to help myself since I am under no delusions that the problem is mine (not his).

I meant that I understand this is my problem, not his

Good start - sort of. It's good that you recognize this as your problem, that you are articulate and seem to be able to understand and want to understand.
The way you describe your love for him is so ethereal and idealistic that I wish you could climb off your cloud a bit. I worry that your expectations are as high as your adulation.
Please understand that there is no Love-O-Meter. You say you never loved the guys in your past, but he loved the women in his. How do you measure his love? He was a player in high school and perhaps that evolved into a sort of easy-loving kind of guy. Easy to love, easy to move on. Then he met you again, and he loves you more and differently, JUST AS YOU DO! Why are you exempt from your past and he isn't?

TRY to believe that. That is part of trust. I always gulp when people say right off the bat 'I trust him implicitly' blah blah and then 'but.. ' You don't trust him! When you imagine her in bed with you two, you aren't trusting him. You are putting her there because you think he is thinking of her. Why? Are you not worthy of his full love?

If you worry that he will leave you because he left them, then you are going to have a very lonely life. People leave all the time for a gazillion reasons. (Actually money is the number one reason, but anyway.. ) Again, this is part of trust. You trust what you have, not what you might have or didn't have or someone else had or what hasn't happened yet. You trust the moment.

(BTW I asked if you wanted to talk about why you are so insecure in regard to your childhood, but you chose to rationalize your fears more instead - comparing your past non-loves to his past loves.)