Complex sentence: The psychological changes are harder to define, because HRT is usually the first physical action that takes place when transitioning and the act itself of beginning HRT has a significant psychological effect, which is difficult to distinguish from hormonally induced changes.
Split into simpler sentences: The psychological changes are harder to define, because HRT is usually the first physical action that takes place when transitioning. Thus, the act itself of beginning HRT has a significant psychological effect, which is hard to distinguish from hormonally induced changes.