Anon, you do not have to forgive him at all. It’s another one of those really quite probematic “recovery” things that people spout without really considering the full implications of what they are saying - mostly because it either worked for them or because they have never been in that situation themselves.
Everyone is different. “Healthy” is not a black and white, yes or no kinda thing. If you think about health like a map, each town is a different kind of “healthy”. And some people live in the “Forgiveness helps” town, while other people live in the “I am Angry about that” town. If you’re not someone who lives in “Forgiveness helps”, trying to go there will not make you healthy at all, and trying to force yourself down that road actually takes you away from where you need to be.
Now, maybe one day you will find that actually forgiveness is something that works for you, or maybe you won’t. Sometimes things change over time and sometimes we stay the way we are forever. And I say this because, there is a trend to invalidate the feelings of younger people and it is plain wrong. Your feelings are valid, and they will be valid whether they change or not. You can’t predict how you will feel in five or ten years, and it doesn’t have any bearing on how you feel now. Trying to force your feelings to change because of that possibility doesn’t help you now at all.
Not to mention, anger is not wrong. Anger is a powerful emotion, and plenty of people harness it for amazing, good things. It’s only a bad feeling if it is used against people who do not deserve it.
So. You do not “have” or “need” to let go of your anger. If someone has hurt you, you have every right to be angry at them, and you have every right to never forgive them for what they have done.