That’s really tough and very unfortunate you have to deal with that :( 
It’s going to be hard but you’ll need to try and let go after a while, and not dwell on that he won’t be as much in your life as he usually would be. This feeling you have for him is really important, and it will be hard to deal with and some times there is seriously no cure to it. You will hurt a lot, and it is worth it because you really do care and love him. If it wasn’t painful, it would mean he didn’t mean much.
When people walk out of your life, new people (or new experiences) will walk in. The past is important, but you need not dwell, to let new chapters of your life unfold and shape you as you grow.
You should try and keep in contact with him, but I’m telling you now, don’t let yourself think too much about how you’re going to wait 6 years for him. Let your mind focus on your life, and living with what’s going on around you, time will pass by itself and who knows where you’ll be in 6 years and how you feel? 
Let yourself cry, let him know he’s very important to you. Treasure your good feelings and good memories with him, but let yourself accept that you can do this, you can move forward. You are strong and our relationships in life are very important. Keep them in your heart with you.