How do you forgive?

How do you go into your head and mentally forgive someone that has hurt you?
I struggle with this, people say all the time ‘forgive and forget’ but how do you do that?
What if something has happened to you that is so mentally scarring and goes against every fiber in your body, you want to forgive the person, but your head wont allow it.

I can forgive trivial things that are mistakes people make in life, but if someone goes out deliberately to hurt you and knows its going to effect you, then a few years down the line decides that because they have moved on a put it down to a ‘bad part’ of their life, expect you to do the same, how would one forgive?

Why should one person (ie the victim) be expected to change their moral beliefs and standards for someone else’s cruelty and then be the one to forgive and move on, if it effects them to the extent its changed who they are for the better?

This is probably confusing to read, but I am really curious as to what peoples idea’s about forgiveness are, and its better for me if I generalise this question rather than be specific.