It's normal to wonder about what could have been but most of the time when people go back to an ex, it doesn't work. Of course, you two were so young that it's not like what you have with your husband now so there really isn't a way to compare the two relationships. The dreams are probably just nostalgia - I think abt my ex-bf from high school every now and again. Think about this: how can you really know that he would be a great guy for you now? I have doubts that the way you remember him is the way he still is today.
People tend to romanticize about past relationships and forget why it didn't work or that it didn't work. It took me a while, but I can honestly say there is nobody in my past (and there's a lot) that I would ever want to be with again. I am not a romantic person so that helps a bit ,but I think I've finally grown up and realized that those men were not for me and I wasn't for them. Many of them were awesome people and have moved on to really great women and have good marriages/relationships. But if I was meant to be with them, I would still be there. Here's a saying that I love and have used before in this forum - "Don't worry about people in your past, there's a reason they didn't make it to your future." I think everything happens for a reason, you and your husband have found each other, it was meant to be that way. It's ok to think and/or dream about your ex, but to be realistic, you're probably over-romanticizing the "way we could be".
Are you COMPLETELY happy in your relationship? Or does it feel like there is something missing? If so...it may be that since that is the only other person you know... like relationship wise... you reminisce on the times you shared together. I am saying this only because... my husband and I have our "problems" and I founf out that he was talking to his ex again who is by the way...married also, with a kid. He had actually been calling her quite a bit. And... when I asked why? He simply said because he felt like we couldn't have friendly conversations. Our phonecalls are always just brief. So maybe there's something you think you might be missing in your relationship. Maybe something the ex did, that your husband doesn't do. I don't know... just a thought. I also sometimes have a dream every now and then about my ex. But it's NOT because I miss him, it's because... hell I really don't know! lol
you're always going to wonder what could have or would have been, what he might have been like etc. that is simply human nature. the thing you need to do is make sure that doesn't consume you. at least in my opinion.