The feeling of being deceived by the person you love and trust the most is like a vicious stab in the heart, and it hurts so much that you can't breathe.
I know it's time to leave, but I'm torn between whether I should simply just delete all contact information and not say anything, or whether I should calm down, talk to the person, and make clear my hurt feelings before expressing my decision that I want to leave.
I'm not sure which way to go, trying to think rationally and worrying about acting impulsively now and regretting it later.
There's also the fear that if you do go to talk about it, you won't be able to control your emotions and say something sarcastic or hurtful.
What the hell am I supposed to do?

Hello! From your description, I can feel your pain and inner struggle.
Being betrayed by your favorite person is indeed a very hard thing to go through. Your ability to analyze the pros and cons so rationally is really remarkable and not easy.
It shows that you are not an impulsive person and that you are still looking for a balanced solution despite the aggression and anger.
There is no standard answer to this matter, no one approach is absolutely right or wrong. Perhaps you need to give yourself some time.
Take a breather and decide how to respond based on how you feel inside.
First, take care of yourself and ask yourself: what do you really want? What do you care about most?
You said."After calming down and having a chat, to clarify the other party's harm to me before expressing the idea of leaving", from this sentence, you may just want to express the intention to leave, not determined to leave, perhaps there is still room for discussion.
That's why you hesitate to decisively delete and just leave, because you're afraid you'll regret it later.
Whether you talk about it or not, there are two possible outcomes that could lead to regret.
In order to avoid regret, it is best to calm down before making a decision and give yourself time to fully think about it.
I'm counselor Fang Yuan, I hope these suggestions are helpful and I wish you all the best!