Re: 1 and 2: Truly beyond our control. It is okay to let go.
I've always had to take care of myself. If I can, I try to help others. Sometimes, they try to help me. Mostly, they can't understand much beyond themselves. On a certain level, I can understand that they don't know what they don't know. Now though, I am coming understand that my energy is finite. No one will take care of me in the end, so I have to plan on taking care of that myself; I can't leave it up to fate, or to other people.
Re: 3: This is where I am taking a little time. I am giving myself space to think about what would make me happy, that I can realistically pursue before CTB. Maybe spend a weekend listening to music? Maybe re-read a book that made me happy? (maybe a few Stephen King books!) Maybe re-watch a movie that made me happy? Maybe sit outside at night and stare into space until I fall asleep? Maybe enjoy sex with someone random? Maybe spend some time taking care of animals at a shelter? Maybe learn to make an amazing last meal, make it and really savor it? Maybe more than one meal? (Thai, Mexican, sushi, Italian, or middle-America-comfort food?)
I already have a few things in place for when I decide that I am done. It could be within a few weeks, or it could be a couple of years. October is my favorite month, by far, and so if not this October, then maybe I wait a year. I might only get one try at this.
There is zero chance of any person changing my mind at this point. (I'm in my 40's now, and I have been thinking along these lines since before I was 10 years old, with no fundamental changes in thinking. Maybe I was 7 or 8 when I was first thinking this way. I can picture it vividly, where I was, what I was wearing and what I had for lunch that day, but I can't pinpoint the exact age.)
Really, the only remaining question is like your number 3: Am I certain that there is nothing else I want to do? That is what I have left to work on.
Lots of people need me, in a technical sense, but I am running low. I feel that I have given above and beyond over the years, and I will do what I can to make the transition easy for them. Re: other people: It doesn't bother me that it is getting close for my time to leave.
Re: my cat. She is very old and not well. I have to stay at least as long as she does. She doesn't like her prescription food. A year ago, the vet said that she would probably only live another year or tow. So now I go fifty-fifty between the limited kind of food that's good for her and the foods that she likes, even though she barfs them up half of the time... she is just so happy in the moment that she's getting it; It's gross to clean up after, but it is what she wants, and I owe it to her. Some day, she won't enjoy it anymore. And then, I want her to go peacefully.
That's what I want for all of us, when our times come.
There are a lot of us, human and animal, in a similar position.