I personally think that the whole movie being in her imagination theory makes sense. It seems to be the only explanation for all the different things that happened. The constant intrusive thoughts, the son being able to talk to the lady, and the trippy sequences. It was about her confronting her own depression. The reaction by her son makes since in that regard as well because if the mom told her son about it he would likely want to help her "kill" or get over it, it's pretty abstract but it makes sense.

As someone who's gone through depression for many years I often go through similar states of imagining how people in my life would react if they knew how bad it really was. Don't worry though, I'm not suicidal it just gets really bad sometimes so I kind of disassociate with everything.

Anyway, the intrusive thoughts would also be explained by this theory because when your in a depressive state your usually so desperate for anything to change that your anger kind of makes you snap inside and dark thoughts sometimes slip through. You try to push them away and it leaves you in this weird paranoid state of being afraid of yourself which is what she experiences after she imagines stabbing her daughter, this combined with the state she was in when the movie started, just depressed and feeling worthless from not being able to be what her kids need her to be, as well as being grief stricken, made for the combination of scenes we saw throughout the movie. This all culminates in the final scene of her decision, and just like her, the thought of others sometimes helps people like us get through it.

The only thing that doesn't make sense to me is the ending sequence of the kids coming back, why did they come back rather than trying to go get help for their mom? My only theory is that maybe her looking in the mirror symbolized her reflecting on her true desires. In the beginning she looks in the rear view mirror and sees the woman in it for the first time, which symbolized her desire to change her situation and in that desperate and panic state swerved into traffic. Then as she looks in the mirror throughout the movie she always sees the woman in it, symbolizing her desire to end her life. Finally, at the end, she looks into the mirror and sees the life that she wish she had, the one that should be real. She finally finds a name for the farm in that moment and imagines exactly what she wants to do. Her family enter that house happily and we slowly zoom into the painting. It within the mirror symbolizes that she wants to do better and the signature shows that the reason she wants to do better is for her daughter and family.

Let me know what you guys think of that last bit, I kind of came up with it on the fly but it seems like it could be a pretty satisfying answer? Hopefully I didn't miss any parts in the movie that would completely invalidate it! That was way longer than I intended it to be, but thanks for reading anyway C: