The character is a woman recently diagnosed with a muscle disorder and has made a promise to off herself on her birthday when her disorder gets "bad enough". More details in the comment portion. Spoilers.

[Christina]

I read this book in 2018/2019 I believe.
It's a book that follows three characters but I skip the chapters about the other characters as the woman has the best story.
The character in the book that I remember was a woman who was diagnosed with a muscle disorder that will eventually leave her in a vegetable state. She has an adopted Asian daughter. She drops her daughter off at school every day until one day the daughter (who I believe is in kindergarten) tells her she no longer wants her to walk her into school. (At this point, the woman's gait is off and it is noticeable. I believe the children in the girls class are pointing it out to her as well). One day in the grocery store, she feels the need to pee. She's looking for a bathroom, everything is getting intense and the need to go is growing. She ends up using the bathroom on herself much to her daughters chagrin. A female shopper and her daughter sees her wet clothes and makes a comment about them, embarrassing the main character's daughter. The woman is so embarrassed about having went to the bathroom on herself that she leaves and stays home for awhile afterwards. She has been going to the doctor for insomnia but in reality, she's been saving her pills to take to kill herself with. (She makes a deal with herself early on in the book that when her disorder gets bad enough, she'll go ahead and off herself). She keeps declining and one day, he husband asks her if he should go for a long or a short run. She has already decided that today will be the day she ends her life. She tells him to run a long one (I believe her daughter is out of the house at Grandma's). She gets in her car with her bottle of sleeping pills and realizes she's left (I believe) her vodka in the family car. She gets in the van, sits down and smells all the scents.. her husband's aftershave/cologne, her daughter's seat, the chicken nuggets from McDonald's.. and for half a second she almost reconsiders going through with it. She does end up doing it though on her birthday (because she didn't want her family to have two days to mourn her..her birthday and her death day)and that's where the story ends. It doesn't go into detail about what happens afterwards (with her family, etc) but I know it was a tearjerker for me and I've been trying to find it ever since I read it.
Thanks in advance!