The closest you'll ever get to a psych study of Caesar is historical fiction, e.g. Colleen McCullough's series, which is fab, but (as it says on the tin) made up, albeit based on years of detailed research.

We just don't have much evidence of what he was actually thinking at any point that isn't either tinged by being aimed at a particular audience (e.g. the war commentaries) or from someone else's point of view. We have very little of Caesar's correspondence (maybe two or three letters to Cicero?).

So the biographer is left to deduce what he can from the evidence. This luminous Roman's mind remains shadowed to us.