Anyone care to tell me what was going on in Einsteins mind after he completed his Special Theory of Relativity?

I have done a course in Special Relativity, understand it and I can see how Einstein arrived at all the equations derived from the theory. Essentially he was wondering what would happen if he was looking at himself in a mirror and travelling at the speed of light. He then came across the null results of Michelson-Morley experiment which tried to detect movement of Earth relative to the supposed ether. He probably knew about the Lorentz-Fitzgerald equation of length contraction. So when he proposed that the speed of light was constant c and physical objects could not travel faster than the speed of light, then using a simple thought experiment (as described in Young and Freedmans University Physics) he was able to derive Fitzgerald Length contraction equation from basic principles. Then the rest of Special Rel followed suit.

I have also done a course in General Rel. and read most of D'Inverno's "An introduction to Einsteins Relativity" a few times since then. But I cannot for the life of me see why Einstein decided to move from Special Rel to General Rel.

I mean like what was going through his head, what did he know and why did he think that Special Relativity was not the full story?

I know he was thinking about a person trapped in an elevator, how gravitational field of earth was the same as an accelerated frame (like if the elevator had rocket boosters attached to outside of it and accelerating at value g) but it just seems like a quantum leap to go from this thought experiment to the full field equations and the equation of geodesic deviation.

Thanks Justin