Would you think my trans* character is selfish?

Well, I can think of lots of questions here.

Any reason why she can't identify as female and be an heir and lead the kingdom? Because of the hormone blockers, I'm assuming this is a modern setting, so daughters should be able to ascend the throne and rule and lead and all that. Are people aware that the king had two sons who suddenly became daughters? Is it feasible to formulate a story that they were girls disguised as boys when they were little (not really a lie)?

And does she believe that a monarchy is a good or bad thing for her homeland? If the social upheaval is because the people want democracy, then maybe disappearing and not pushing her claim to the throne could be seen as a good thing.

Who is main character 1. What does he or she think about the situation? What kind of relationship does this person have with your princess character?

Is there any reason why a transgender person can't have the same kinds of conflicts between duty and personal desires as a cisgender heir to the throne who has to choose between a quest and managing things at home. If you want to emphasize selflessness as a character trait, I suppose you could throw in potential greater good versus greater bad outcomes for each choice and examine the inner conflict of the main character as she worries whether her choice is driven by her own desires or by her concern for the greater good.

I'm not an expert on the negative stereotypes that plague trans characters in novels, though.

If she is doing the most right thing possible given the information she has, then that is not selfish. You could however massage the plot to make this less ambiguous if you want. e.g. If the monarch must be able to produce heirs, and she is leaving to bring back to life a sibling who will be able to do this -- for example if a spell will allow the sibling to come back as a fertile female.