Is it permissible to kill someone who sexually assaulted me, and what is the way to forget?

What you did was a terrible sin and an enormous iniquity. You should have denounced this evil instead of going along with it. Repent to Allah with sincere repentance, and He will erase your sin, for Allah forgives all sins for those who repent. The one who repents from a sin is like one who has no sin. Every time you remember the sin, counteract it with effacing good deeds and acts of obedience. You are not permitted to kill this wicked person who committed such an act, because the implementation of legal punishments is solely for the ruler, and you are a partner in the sin. Focus on reforming yourself and avoid this person.