This is exactly what it's like to grieve the loss of someone you care about while desperately trying and failing to not blame yourself: living in what is explicitly your worst nightmare, a (metaphorical) dead body following you around that will kill you when you let it get to you, escaping into your own head when things get tough and imagining a world where they're still alive, a self-imposed torture cycle that you refuse to break because maybe this time when you punch the wall it'll fix everything, only for the dead body to reach you anyway. For (what feels like) more than two billion years.
When i first watched this in 2015 i just thought it was "doctor who monologues at the audience (extreme mode)" but now that time has passed and things have happened, this episode is far more resonant and deserves all the praise it gets. Stevieboy you may have some glaring writing problems sometimes, but other times you really nail it.