Hi dear.
I guess I don't quite get your title question. Are you asking why he's going to Korea, or why he is letting you know he needs his space?
I also assume from your avatar that you are a bit familiar with the military side of life. So, maybe your main issues are the five questions you listed at the end of your post.
1. Of course he's going to think about you, but his military duty is going to keep him busy too.
2. Maybe he will miss you - that depends totally on him and you have no control over it.
3. What exactly has he done to you? You wrote 'ex fiance' well that means he broke it off - but you did not state why and how long you had been together or engaged - or what happened to cause this break - unless you wrote it in another thread, which makes it difficult to keep up jumping from one thread to another - the same issue should always be kept together so that we see the whole picture.
4. The only one who can answer if he will call you is not on this forum right now, and you should be asking him.
5. Again, only he can answer this one. He just might need some time to collect his feelings and make a decision whether he does want to see you again. This depends on the circumstances of your relationship and that is something that you will eventually find out when the time comes. But, again, he's the only one that has control over this, so leave him be for a while and don't pressure him. He needs time to settle and adjust to his new duty station and will probably be busy receiving orders and instructions while inprocessing.
Being familiar with the military, you know that they will always have to be in a state of 'readiness' in case something crazy happens in the world and they get deployed somewhere else. So, nobody can guarantee that his tour of duty will be easy for him or not. Just give him some time to adjust and cope with his current situation and try not to be a main stress-factor right now.
No soldier's life gives them a chance to plan a certain future right now, and some might think it's easier to cope alone without having to break a promise to another person in their lives. Maybe he just does not want to burden you with an uncertain life and might not want you to press for answers or promises that he might not be able to keep, so give him a breather and when he's more relaxed and has a chance to plan a few certain things, he just might include you in these plans again.
As for you being able to find someone you can count on and trust, and open up to, - again, if they are in the military, there are no guarantees there at all unless you learn to accept that, you will probably not find what you are looking for in that area. Not even police, firemen, pilots or security guards in the world today have a certain and guaranteed future. But, they never have a boring moment, that's for sure.
You might want to look in an area of less daily dangerous excitement, such as doctor, mechanic, nursing, farming,etc. But that is all up to you.
No matter what your choice, I wish you all the best of luck and hope that you get the opportunity to share a good life with that someone special.
Keep in touch. Check out learning how to throw a bowling ball, play tennis, or jog through the park - maybe you'll find some new friends there.