godnotgod
Thou art That
But that isn't just training, it's a product of language. In the English language, a verb demands a subject. Quid pro quo.
I don't think Descartes is to blame. Rather, the very nature of English grammar.
The language is part and parcel of the social indoctrination*, first to establish the self, and then to reinforce it throughout one's life. Whatever system of reward and punishment the applicable culture has in place galvanizes it, even after death.
Descartes may not be to blame, but his logic was mistaken. However, the whole idea of existentialism must have come about within a larger social context/paradigm which nurtured it.
BTW, Zen would jokingly ask: "So when not thinking, you don't exist?"
*For example, we say 'It is raining', when there is no such 'it' that rains. Or we may think a tree is made of wood. It is not. A tree IS wood. We call a body of flowing water a 'river', but no such 'river' exists; there is only flowing water. Same for a 'whirlpool', and ultimately, for the 'I' that thinks.