What is the ruling of Islamic law regarding how to deal with a person who sexually assaulted and harassed the speaker years ago in the UAE? Is it permissible for the speaker to seek revenge on him, or should they resort to the local or Canadian authorities? And how can one get rid of the negative psychological effects of that incident?

What you have gone through is severe injustice and harm, but professional boxing is not a religiously permissible solution for revenge. Retribution (qisas) must be equivalent, without exceeding the original offense. If someone violated your honor, you are not permitted to violate their honor. These matters require a just authority to implement the prescribed punishment, and this is currently impossible for you due to the distance between you and the aggressor. The verse from Surah Al-Hajj/39 does not apply to your situation. Furthermore, seeking victory for the oppressed is conditioned upon not exceeding the wrongdoing in punishment. Allah Almighty said: "And if you punish [an enemy], then punish with an equivalent of that with which you were afflicted," and "The recompense of an evil deed is an evil deed like it." Similarly, punishing this oppressor is not possible for you at this time. What is incumbent upon you is to consult a psychiatrist to overcome the effects of the psychological trauma. Allah has permitted the oppressed to supplicate against those who wronged them: "Allah does not like the public utterance of evil, except by one who has been wronged."