Emotional cheating...

Okay so this is going to be a bit long but I need to vent and get some advice. I started dating my now husband when I was 16 and he was 17. We have been super close and connected from the very beggining of our relationship. He had a best friend named Mackenzie since he was practically born. There are pictures of them in diapers together, in preschool together, all that nonsense. She was always super nice to me and I didn't feel threatened by her at all. Which, this sounds a bit mean lol, she wasn't super attractive either, so I didn't think I had to worry. Well, then I got pregnant about a year into dating. We NEVER talked about splitting up and he never showed any signs of "freaking out" or leaving me. I felt 100% stable. He moved in with my parents and I because his mom is a nut case (different story lol), we had our son, and he proposed to me. It was PERFECT. He got a good paying job, was super helpful with the baby, all was well. That's what I thought anyway. Well, 10 days before our wedding we are over at our friends house and his phone goes off. I pick it up and look at the message. Not to snoop or anything just because I was going to tell him who it was. It was Mackenzie. All she said was "I miss you". Something in my gut just told me to look at their conversation. I only read like 4 messages. One of them said "hey sexy been thinking about you" and the other texts were along the same lines. There was never anything about them being physical or meeting up...but it still absolutely DEVASTATED me. I mean it was like my first heartbreak all over again. I walked out and just started bawling and he comes out to the garage and askks me what's wrong and I told him what I saw. I asked to see the rest of their messages. HE DELETED THE REST OF THEIR CONVERSATION. So we talk for awhile and he tells me that they have always had a very flirty relationship and that nothing happened they just talk like that to each other. I asked him to completely cut her out of our lives because we have a child and are getting married. He agreed and he has kept his word. He has not talked to her or seen her or ANYTHING in 3 years.. here is where my issue is. I still have nightmares about this. I still dwell on it. Not all the time, but sometimes it hurts so much I just want to crawl into a ball and never get up. He has been amazing and ever since we 've been married he has without a doubt been 100% faithful to me and we are so much closer than we were. So I don't want to bring it up 3 years later but I have so many questions. I never flat out asked him much about it but now I feel stronger in our relationship so I feel like I have the guts to really question him. I have tried so hard to let it die but it won't go away. What do I do? I want to ask him if he LOVED her, if he ever slept with her, if he was talking to her the entire time we were together until we were married. I have a lot of questions. And none of them would make me want a divorce or anything I just NEED to know. I have to get some kind of closure. Should I bring it up or just live with it? Will it ever go away? He's been so great so I hate to put a damper on how wonderful everything has been...I don't want him to dwell on his past mistakes. Ugh. Lol please give me advice ladies. And no divorce or whatever is never and option. He is not a huge douche bag anymore. Lol