We have to put my cat to sleep on Monday. How to stop crying?

We’ve had her since I was seven. I’m twenty one now. She was one of the cats I grew up with, and she holds memories for me of my childhood friends. She was given to us by my terminally ill aunt, and I really don’t want to let my cat go. I feel like she’s the last tangible piece I’ll have of my aunt when her time comes. I can’t stop crying. Any advice?