1) Hank Reardon wouldn't tolerate a life of (self) deception; knowing Dagny wanted Galt, and accepting that truth, was the only internally consistent course available.
2) Hank Reardon loved and respected Galt. If he had to give up Dagny to be able to continue to be a part of Galt/D'Ancona/Dagny's life, then his rational response was to do that.
3) Hank Reardon embodied the self-confidence that allowed him to see Dagny's preference for Galt was not a rejection of Hank, nor did that diminish Hank Reardon (e.g. your value isn't shaped by others).
"My characters are persons in whom certain human attributes are focused more sharply and consistently than in average human beings"
I agree with you on the Rearden vs Galt issue though...that doesn't make sense.