On paper, yeah, it looks like you are overracting. But a couple of things come to my mind.
First, you have been hurt in the past. When that happens, when we get back on that horse so to speak, it can be hard to completely let go of the pain and hurt we suffered in a previous situation. I think with some counseling, it could get easier. But you may just not ever be able to fully get to that point where you can say "oh this doesn't bother me at all".
Your bf may be underreacting to your pain. Which again, is coming from his background of being a trustworthy guy. Had he been the one who had been cheated on in a serious relationship, he would probably feel the way you do if hte situation was reversed.
Its wrong for you to snoop but you know that. To me its not a deadly sin kind of wrong but rather just one facet of what you are going through. I think you should own up to that, tell him what you have done and tell him you are wanting to get some help to deal with what you are feeling.
I also have to say, this kind of situation always has potential to stir up feelings. I have a friend, he and his wife are good friends of me and my husband. This guy and I email sometimes, we all do (the four of us) regarding things we have coming up like camping, dinners, bbq's that sort of thing. So lots of times its group type emails. However, sometimes he and I email back and forth, we both feel identical about current event type stuff, so sometimes he'll forward me stuff and I'll respond back, we are both smart a** type people and we crack each other up both via email and when we all hang out. Perfectly innocent. I didn't think much about it, but one night his wife (who is my good friend) said something that just gave me a little bit of a hint that maybe she doesn't like that. Following that conversation, I had planned to just sort of stop responding to anything that wasn't addressed to her as well (you know, a "hey what time are we leaving to camp this weekend sort of email between the four of us). But he hasn't really sent anything other than that anyway. Don't know if she said something to him or if he just picked up the hint too. I talked about it with my husband and asked him if that had bothered him (its our home email so I am sure he had read some of our banter) and he said no. But I realized that if my husband and my friend were emailing like that, it might bother me becasue I tend to be insecure. I don't think we crossed a line, but if his wife does, even a tiny bit, that is the last thing I want. Her feelings matter more to me and I also had to think about how I might feel about that or how my husband might feel.
I guess my long winded reply is that for many of us, this kind of thing is touchy. You have baggage on this issue and to me its very understandable that you would be upset by this. Its not logical or reasonable on some levels, but to me its valid for you to feel this way. Time will help and I think talking to a professional is a great idea. Because even if this relationship doesn't last (for this or any other reason) this will simply come up again in the next one. In today's times of email, facebook, the workplace, etc, men and women come into a lot of contact with each other. So this isn't a unique situation that can't possibly come up again in your life.
Good luck, I hope it all works out and I bet it will! Admitting something is a problem is half the battle. So you are off to a good start!