Ok so when I finally thought I will finally ask her out, I noticed she spends more time with one of my friends. I was suspicious at first but I thought I was paranoid and jealous.Turns out today, they are actually dating. About a week ago we kissed, so I'm very very confused. I also told her I'm letting her go, because it is clear I do not belong in her life at all. She got mad, told me to piss off, and it would be better if I don't look for her anymore.

All of this is breaking my heart ,honestly. And I don't know what to do, Confront her? Or just let her be?

You leave her alone. Find a new crush. One who isn't dating a friend.

Let her be.