Hits like a truck, good job!
Your imagery does wonders for the message, and this is very well written.
I saw you were talking about the four walls - I think you should keep it as is. I think it has two main strengths - by making it a car and not a room you make the wall fully metaphorical. This adds a hint of constriction, which I think fits well. It also makes the car into more of a room, even raising the question if after what happened with this loved one the speaker moved to their car.
The third stanza feels paradoxical. Almost by definition, by speaking about the last instance of saying the name and remembering past love, this means the day has most likely not come. To me this reads as the fear of the speaker being somewhat detached from the actual state. This might be what you were going for though.
The ending is a bit weak in my opinion. You have a great buildup in the fourth stanza, but the last line lacks punch in my opinion. The whole poem shows unwillingness to go with the natural flow of moving on, so it all being boiled down to "the choice has been made" feels wrong. Maybe if you contextualised the choice as something more nuanced it would have greater impact. I'd recommend tweaking it a bit and looking for something more impactful.
Last thing, I think most of the poem reads clearly as death of a loved one, except the second stanza. I'm having a hard time putting my finger on exactly why, but to me the second stanza reads like a breakup. The duality might be something you want, but I thought it's worth mentioning.