How do you let someone go?

My Aunt and Uncle just left my house, for the last time before they move to Florida forever. :/
My uncle has an incurable illness, and he’s getting worse. The doctors don’t know how much longer he has, and the cold is bad for him, with his disease. That’s why he has to move.
I just don’t know what to do anymore… I know I’m gonna have to say goodbye forever soon, and I might not see him again before he dies…
How do you let someone go? How do you know that it’s going to be alright? How do you feel okay about life again when that person is gone?