How to cope with my cat being put down?

My cat has lived for seventeen years. We almost share the same birthday. She's been there since I was two months old. She's sweet, and tough. She had definitely seen a lot more than most domesticated cats have. She goes outdoors all the time and hunts mice. One time I saw her at a grocery store. She loves to go places, or used to. She would always come home. She slept in the same bed as me, spent time with me, let me cry into her flank when I was upset. She was what helped me cope when we had to put my dog down a few years back. We recently had to put my other cat down in February, because she was getting too thin and her body was shutting down. My current cat was limping and holding her front paw up, and it was very swollen. So about a month ago my mother took her to the vet and found out she has a cancerous mass in her paw. It's getting worse, killing her. She's getting very skinny and her paw is getting worse. We've been giving her shots for the pain but it doesn't help much. One time I heard her meowing alone, at night. I know she's in pain and it's time. It's so horrible to watch her wind down from rabbit-hunter tough kitty to old and frail. She is mentally happy, purring constantly and always asking for attention. But I just know, you can see it when she tries to go down the stairs and falls down when trying to jump on my bed. She spends most of her time upstairs sleeping, but she used to always be outside or curled in a ball in my room. Or eating, she still loves to eat but can't seem to gain any weight. She's wasting away and I was petting her last night. I can feel every bone in her body. So my mom called the vet today and they're putting her down in a couple of hours. I'm miserable, and I can't get another cat for a while at least because of my dogs. How do I cope? Thank you so much.