Madison. PLEASE don't be so hard on yourself! You were not negligent, you didn't kill her, and I seriously doubt that holding her in different positions affected her at all. She was a miracle kitty. Her surviving for 4 years after a lymphoma diagnosis wasn't just lucky, it was the result of your love and care. And I know that watching her spasm was incredibly difficult for you - my Polo had awful seizures today, the worst I've seen, before we arrived at the vet. I don't even want to think about it, and I wish there was some way I could forget them, but I won't, not ever.
I worked for a high-kill county shelter for years. We had a beautiful long-haired tortie named Dixie. I was trying to get rescue for her, but was unsuccessful. We euthanized on Tuesdays, first thing in the morning. One Tuesday morning, I noticed I had a voice mail, but didn't recognize the number so I didn't listen to the message until later. Dixie was euthanized that day. Later in the morning, I finally listened to the message, and guess what, it was a rescue calling to let me know they'd be there that afternoon to get Dixie. I felt horrible, like a murderer, if I'd only listened to that message she'd still be alive. There's a Dixie Street in the town I live in, and to this day, ten years later, I have to avert my eyes when driving by it. I'm 49 years old, and have had many, many pets in my life. I've also worked in reptile conservation and at a major zoo. Many times, too many to count, I've believed - rightly or wrongly - that I was somehow responsible for an animal dying, or getting sick, or not being adopted. All we can do is our best, and that is what you did. Mia loved you just as much as you loved her, and she doesn't blame you. No way.
"Some of you say, "Joy is greater than sorrow" and others say, "Nay, sorrow is the greater." But I say unto you, they are inseparable. Together they come, and when one sits alone with you at your board, remember that the other is asleep upon your bed." Khalil Gibran