Conatus is the actual essence of the thing
Formal Statement
The endeavour wherewith each thing strives to persist in its own being is nothing other than the actual essence of the thing itself. From a thing's given essence, certain consequences necessarily follow, and things have no power beyond what follows from their determinate nature. Therefore the conatus is not something added to the essence — it is that essence in action.
In Plain Language
This is the step that turns conatus from a physical metaphor into a metaphysical principle. Your striving to persist is not some feature you happen to have, like being tall or having brown eyes. It is you. Strip away the striving and there is nothing left to call a self. Essence is not a static blueprint; it is a dynamic power of existing and acting. When Spinoza later defines desire, joy, and sadness, they will all be variations on this single theme: the fluctuation of your essential power.
Why This Follows
From ce-07, everything strives to persist. Spinoza now shows this striving is identical with essence by noting that a thing's power is exhausted by what follows from its nature (I.P29, I.P36). There is no surplus power and no deficit — the striving is all the thing is.
Conatus is not a property of essence; it is essence itself, understood dynamically.
Connected Concepts
If your essence just is your striving, does that mean you are a different "you" when your power of acting changes — or is it one essence with variable intensity?