By getting over yourself.
No one did you any harm.
You're 20 years old and have much less experience in relationships than those who have been around the block a few times. More than likely you will not end up spending your life with this person, and will have the opportunity to have good sex with multiple people that you at least respect.
(almost) Everyone has sex. In future years, you will have the experience of all the things you've done sexually with her, and vice versa. What she has done with someone else, or what you are doing together is not unique in any way. Billions of others have done exactly the same things you have done, and much more.
She didn't "betray" you. She went out with someone else in the past, when you to weren't seeing each other.
Was she suppose to somehow know she was going to meet you/get back together with you, and keep herself from living her own life, but instead live for the future you? Get real and grow up.
You say she "didn't even like him". Well, she liked him well enough it seems, and that's none of your affair.
This is your opportunity to understand the world doesn't revolve around you, that no one is doing anything to hurt you, and that what has happened between the 2 of them is absolutley none of your business.