I understand Thymology to not merely be a synonym, but to be a branch or area of Psychology, however I am at a loss of how to properly distinguish the two.
What distinguishes Thymology from Psychology in general?
Thymology is an historical conjecture on why someone thought the way they did. For example, why did Herbert Henry Asquith take his country to war in 1914. What thought processes went into that decision. The decision is often made to seem inevitable given preceding experiences. There is a school of thought that would lump this study alongside other accounts of history and journalism, in that they fall into the trap of searching for causes for things that are just as likely to be caused by randomness (i.e. perhaps he went to war because the decision was made shortly before lunch and he was hungry and irritable, rather than because of a neat causal explanation).
Psychology is the general study of mind and behaviour in general, largely in the present, and tested scientifically with controlled experiments, simulations, statistical inference, and qualitative research.