I think that if you write something, you need to know ahead of time that they are unlikely to accept what you wrote has validity in it.
There are adults who have reared children badly and have come to realise it and are sorry for it. But those adults actually listen to their adult children and listen to the pain and apologise for causing it. Often things can be mended then (not always, but often). Your parent hasn't - they've minimized it.
For completeness, it's worth writing down what actually happened in factual terms. Depending on your parent, it might be worth stating how this has affected you - again, factually. Oblivious / emotionally unintelligent parents might just realise how they've affected you if you do this. But malicious / unkind parents will get off on reading about your hurt. So think carefully if your parent is either oblivious or unkind. Don't open yourself up to more hurt to the nasty sort.
Sit on the letter for a week or three and then consider again whether to send it.
What do you hope to achieve by sending it? What will you do if your parent denies the events / atmosphere / what you've said? What will you do if they show some glimmers of understanding? What will you do if they genuinely apologise and try to rebuild a relationship?
Writing the letter, sent or unsent, can help clarify your own thoughts. If you do send it, you can also say to yoruself that you have absolutely explained to your parent why you're stepping away. You've done everything you can to mend the relationship. But as I say, consider carefully what sort of parent they are, and what you hope to achieve by sending it.