Damn. That was heavy. If any philosophy is gonna cripple your will and give you an existential crisis, it's this.
A fair amount of this went over my head and I definitely did not understand all the nuances of Schopenhauer's arguments, but I think I understood the broad strokes. In brief, this is what I gleaned:
There is a singular, blind, universal Will that represents itself as the entire multiplicity of objects, sentient and insentient. That includes everything in the entire cosmos that is, has been, and will be.
Schopenhauer claims that we misidentify ourselves with a limited object rather than with the universal Will which he regards as the underlying reality.
This underlying reality is in itself changeless but its representations are continually changing.
Because the Will does not recognise itself, it takes itself for the representation rather than 'the-thing-in-itself'. Therefore, the Will fears it will end, hence the fear of death.
The Will can recognise itself and be at peace by seeing through the illusion of the principium individuation - the belief in being a separate entity.
The more the Will recognises itself, the more it treats its various forms as though it were itself. This is because the Will recognises that what it perceives as 'other' is actually itself, and so, to do harm to the 'other' would be to do harm to itself.
I found this mind-blowing, exciting, and, believe it or not, miserable.
Rather than presenting Oneness in an optimitistic new-agey light, Schopenhauer presents it as unbelievably tragic. This is because Schopenhauer claims that suffering exceeds happiness and it would be better to deny the Will to Life, ending the suffering of life by discontinuing reproduction. I don't agree, but Schopenhauer makes a pretty convincing case.
Although I did somehow enjoy this, I would be remiss if I didn't mention the chapter 'On Women'. The title itself was a huge red flag and I couldn't bring myself to read more than the first paragraph - it was misogynistic drivel and was completely out of line with his ethical theory. I don't think treating women as inferior to men is in line with drawing less distinction between yourself and others.
As a whole, I read this as Schopenhauer's attempt to present a philosophy that is so pessimistic that it breaks the reader's illusion of an individual will, causing the reader to recognise their identity as the universal Will to Life, thereby collapsing the illusory distinction between the perceiver and the perceived. I think this is an interesting tack as a spiritual teaching but also potentially problematic.