As some of the other users have already replied, I think that Alaska's
last moments at this point
in the book were not necessarily dominated by sadness. Panic, guilt, desperation, maybe, but not simple sadness. Also, if I remember correctly it is left somewhat ambiguous even in the mind of the narrator whether or not Alaska actually
killed herself, or if it was a DUI accident...
...you seem to have decided on the former. Maintaining that ambiguity in the first person would be difficult but not impossible, but would also be fine if your chapter resolved it. I think important keys to focus on in your chapter might be Alaska's emotional thoughts about some of the pieces of her past that have brought her this painful sadness, and loss of others that brought her joy. These thoughts might play well interspersed with glimpses of the twists and turns in her physical surroundings.
She may feel alone on that winding road at night, as alone as navigating the labyrinth. To show how thoroughly sadness/guilt/regret has consumed her, display detachment from the real world -- her driving might be sloppy and things seem to sneak up on her, she grows more and more lost until she comes around a corner and some flashing lights bring her back, and she makes a choice. I think her hands gripping the wheel, tears and sweat sliding down her cheeks, the sweet aroma of the flowers fading as she cries and her nose gets stuffy are examples of good things to show. She might think of her mother/her mother's death -- The flowers in her seat imply she was on her way to deliver them when she crashed, that remembering and honoring her mother was at the forefront of her thoughts when she got in the car. Her Life's Library or whatever it was called -- she probably feels some regret at things left unfinished. Her friends, and also, from that, all those last words. Does she say anything, as she slams her foot on the gas? It might be touching and ironic if her last words were somehow related to Miles/Pudge.
Based on her dialogue and actions during the story, I think that her thoughts, her narration, particularly that to convey unbounded sadness, would be erratic, careening and spotted with moments of clarity through which she recalls the past. She's a very spontaneous, bold character. I'm not sure an emotional description is what's needed here, because she may not be self-aware enough to describe how she's feeling, just feel and do.