I feel for you, I really do and I don't think there is an easy way to cope other than to tell yourself you will see them soon.
I spent from Feb to May in the US with my little granddaughter aged two. The night before I left, she dropped off to sleep on my lap whilst I was reading to her, and I cried as if my heart would break, knowing that it would be a long time until we would meet again. I couldn't have handled having to say goodbye at the airport, and left while she was still asleep. I did, however, leave one of those recordable story-books behind on my bed for her to find when she came looking for me, so she could still hear Nana reading to her. Needless to say, am counting the days until we can visit again.