There's been a lot of good advice given. I don't really have much advice, as maybe a different perspective on a couple of points you've raised:
-Regret. Yes, you may regret not giving Adam another chance, but you might not even realise what you may miss out on by giving him another chance. Even if you don't get into an exclusive relationship, LDRs by their nature isolate the participants, at least in my experience. This is hard to manage even in relationships where both participants are working towards the goal of being together in person.
Even if you don't formally resume an exclusive relationship, you may shut out the possibility for a more fulfilling relationship because you feel an obligation to Adam. While I think things generally work out the way they're supposed to in existing relationships, I don't think that's always the case in relationships that never are started or are stunted early in their development. You might end up regretting never persuing other relationships if the one between you and Adam doesn't work out.
What I am trying to say is, regret is not always predictable. Avoidance of regret isn't a good reason to stay with someone.
-What you want vs. what he wants I am not sure what Adam wants because I don't know the full story on this one. From your post you clearly want marriage, kids, and a lifelong partner. They are your needs. You can't get these needs met without Adam having similar wants and needs even if he isn't 100% sure he wants to marry, have kids, and be a lifelong partner with you. If it's not in his nature to want this, then it probably will never be. This is about the only direct advice I am going to give you. If he doesn't have the same goals in life you do, especially when it comes to family and marriage, you are wasting your time or setting yourself up for colossal heartbreak. The heartbreak you feel now will be minuscule compared to what you may potentially feel when you've sacrificed your core needs and goals for someone.
But if Adam does have those same life goals, why the caution? Is it because he feels he's too young or because he is afraid your relationship is too untested? Or something else?
Did you give him an ultimatum? Like make some commitment to our future or I am leaving you? I hope you do not feel guilty about that. That isn't forcing him to do something unless there was manipulation involved. If you were assertive and not aggressive or passive aggressive, then there is nothing to feel guilty about. I am hoping for the strength for you to stand by this ultimatum if you gave it. Healthy people usually don't use them unless there is good reason, and it's not forcing anyone to do anything. You are getting your needs met while respecting Adam's rights. He does not have the right to expect you to wait forever for him.
-Being friends with people you were once involved with I've never seen this work out without someone being hurt or there being some sort of weird uncomfortableness going on. I think it also helps delay dealing with the loss of the relationship. Sometimes, when there are kids involved, you have to at least attempt to make this happen, but it's a weird area. Personally, the only time I've seen it sort of work with people I know is with my best friend and her ex who ended up dating a man after her. Still, when you see the two of them together at parties or whatever, there's a lot of negative tension. When she complains about him, it's the same stuff she complained about when they were dating. But they are civil together and social. I don't think they turn to each other for support or advice though. I think a big chunk of what makes a friendship worth maintaining is missing for them. But if you asked them, they'd probably say that they are still good friends.
I thought I had cracked this problem with my ex, but Mr. Moggs got upset that we still talked occasionally. When it came down to it, the friendship with the ex wasn't worth even a little bit of uncomfortableness on Mr. Moggs' part, so I ended it. "We can still be friends" usually has the best of intentions, but even when said in sincerity is a very hard road to go down. But who knows, maybe with the geographical distance, it might work in your case.
-You miss him This is only natural. Whatever you decide, I am hoping for the best for you.