You're not alone. Part of being in a relationship with a person from another country is having to make huge compromises and the fact that one of you is going to have to make a huge sacrifice to move away from everything you know and start over.
I wish I could say that it all gets easier but I'm not sure that it does-you might get better at dealing with it or one of you might find it easier than the other, but there are always going to be things that crop up-job loss, death in the family, illness-that throws everything out of balance.
The key, in my opinion, is to be flexible. If you move back to America thinking that that's a decision for life you may find that it is not. He might not like it, something may happen in his family or job that make it better for you to live in the UK. And if you cling to the 'this is forever' argument that's just going to cause trouble.
But you're right. There's no point worrying about what's going to happen in the future. There is absolutly no way you can predict it. And worrying and fretting isn't going to do anything except cause tension.
So, I think the best thing to do is to sit down with him, and really honestly discuss where you both think the marriage is headed. Put it all on the table about how willing you both are to compromise and try to get an idea if you're on the same page. Because if you're thinking that it's a permanent move and he's thinking it's a trial it's probably a good idea to discuss this now, before you book the moving van.
But most importantly, I think you need to focus on where you are now. Find things that are positive and focus on being happy here and now and not just thinking about how rosy the future will be.
No matter what happens, you'll be fine. It is hard, but you know that you can do it. A lot of us have felt unhappy and missed our families and I know how difficult that can be. But it is do-able. You just need to think about being happy in the present and letting the future unfold as it will.