I know that feel, dude.
Just lost my grandmother in March, literally the day after her birthday. I busted my ass to draw one last thing for her for her birthday present and she loved it.
I'm getting married on Saturday and it hurts to know she won't be there, but at the same time I wanted her to go in peace which she did.
All I can say is, spend what time you have with her learning about her young life, try to tell her jokes or stories to keep her engaged, and just generally help her realize how important she is to you.
We unfortunately don't get to keep our loved ones forever, so we have to cherish the time they have with us. Reminding them of what they mean to us, and showing them that we've listened to them and learned from them is one of the best gifts you can give to someone who is ill or passing away.
When you're away from her, look yourself in the mirror and think of everything you love about her, everything she's taught you, and everything you're grateful for. That's what has helped me.