Hey OP. I'm so sorry you're going through this.
I've purposely not RTFT as I think the replies will make me cry. I get very upset with this sort of conversation.
Having your darling pet PTS is so, so hard. Like you I was teetering on the edge of was I giving up too soon? / prolonging her pain for my own selfish reasons of not wanting to let her go?
My girl was 17, she'd been ill for three years. But still hassled me for walks and fuss, so still enjoyed life and carried on.
She started having fits too, each one longer and taking longer to recover from. It was heartbreaking to witness as I held her until it stopped and she was back with us.
One day she had a huge fit in my arms and stopped breathing. I was so shocked that I shook her, screaming. She coughed and slowly came back round. But the spark in her eyes had gone. I knew right there she'd now had enough.
Making the call was so hard. Couldn't get the words out, the receptionist guessed. Booked it for that afternoon before I could change my mind.
We paid extra for a home visit as the surgery only had bad connotations / memories for her. Didn't want her to be stressed.
Next four hours until vet arrived, we cooked some steak and fed her that and cheese, groomed her, cuddled her and talked to her.
The vet put her to sleep in the garden she loved, while we held her and whispered into each ear a big thank you for being our dog for 10 years.
That was 10 years ago. I've two more rescue dogs now, aged 10 and 12.
I'm welling up typing this. But what I'm trying to say is that you'll know when it's time and to consider a stress free experience into the next world if you can. As much as we miss our girl, her passing and the hours leading up to it were just right, in the circumstances.
Good luck. You obviously love your dog very much. This is the hardest part of having pets.