Goodbye so soon?

So I'm in Phoenix right now for my Greatmother's 90th birthday. 90. Wow.

I'm excited to be here because my whole family is together and it is going to be fun. But tonight, my mom and I were hanging out in the hotel spa and she told me that she wants me to use this weekend to say goodbye to Greatmother.
.
.
.
.
.
I was not prepared for that.
.
.
.
.
.
Is this really the last time I'll ever see her? I don't make it to Phoenix all that often...but really? How do you say goodbye to someone you love so much when they're not even gone?

I don't really know how to do this.