I was with a guy in the army who was always based at least 4 hours away, if not more. Quite often his weekend leave would be taken off of him which was heart breaking but we managed it for almost 2 years. Out relationship broke down a month after he had spoken to his mum about proposing, however we broke down due to my mum hating him and that he didn't want the relationship to change as much as I did. The distance didn't break us, we just had very different lifestyles and he had other things which he was more focused on.
It's hard, really hard, but whenever the distance got me down when we were together I just used to ask myself whether I'd be happier without him or count all of the things about him which make me happy