Don't like to say good bye

I met a lot of friends and students in Vietnam.
This is a good thing.But I also have to say good bye to some of them.Students have to go to Japan, colleagues have to transfer another branch office. Of course I understand that it's necessary.But when I say good bye, my heart is full of sad.
Today, I heard that one of my best friend have to go to another office.When I heard this thing, I felt sadness, loneliness, and annoyed.I couldn't control my mind.I know that It is not her fault, but I couldn't say anything to her.

I'm sorry today, and I promise.When the time is coming, I will say good bye with smile.We're friends wherever we are.