She (22F) cheated on me (21M)

I'm devastated thinking about it, but i need someone opinion.

My Gf (we have been togheter for 9 months) came to my city for 12 days during summer. During this time i let her met my family and my friends, which they immediately welcomed her. In particular, the guy who was my best friend really got attached to her.

At first i didn't think nothing about it, i was only happy that my friends and her got along very well. Then we did a campfire on the beach with all my friends and her, and at first she and him wanted to swim, which was a good idea but they seemed too excited to do it, and said only "we are going to swim", that didn't seem like an invitation but we all followed.

Later that same evening they starter pouring their drinks on eachothers head and used the excuse that they needed to clean themselfes, that time nobody followed because we didn't want to do another bath.

Even then i trusted them because why did I have to think badly?

Then 2 days later, still all my friends and her got to a open-air disco, we were having fun. My Gf and my female friends went to the bathroom, after a while my Best Friend got up and said that he was going to search them, i was about to follow him when he said "you stay here and keep company to X (another friend that was with us)". I didn't think nothing about it because, you know, i trusted them. Then my female friends came back, but not them. After a little bit more they came back. Even then, i didn't think nothing about it.

The next day my Gf was so sad, she didn't want to get up from the bad, when i asked her why she said that she was sad because the vacation was about to end and i didn't ask her anything else. She went away, kissing me goodbye and the same day also my Best Friend went to a vacation with other friends for a week.

During this time my Gf was strange but still she wouldn't tell me the truth.

It was only the day when my best friend had to come back that my Gf called me. At first she told me she wanted to break up for a discussion we had before she arrived, and then, only after i was taking my responsability and genuinely told her i was sorry because i knew i wasn't the best boyfriend, she told me that the evening at the disco her and my Best Friend kissed, telling thing like "you know how much i appreciate when people remember things" and "He made me feel understood. Nobody ever made me feel that way".

Now it's been like 10 days after. We have still to talk face to face but i keep changing ideas every 2 seconds.

For now i think we could never get back together, because i know i can't trust her and this would hurt both me, for obvious reasons, and her, because i know i would never treat her the same way.

She texted me how much she's sorry and how much i mean to her, that the kiss meant nothing to her and that she hasn't any feelings for him. She tried to say that it was "only a mistake and nothing more" and that she will go back in therapy and she will make things right this time.

I'm hurt, she's hurt, i know i can't keep staying with her but leaving is so hurful.

What would you do in this situation?

I'm really sorry for my bad english

P.s. for who's wondering about my old best friend, i told him i never want to see his face ever again, he tried almost to say "i will get you back one day, i swear" but it only made me want to puke.

tl;dr my Gf cheated on me with my Best Friend and now she's hoping i'm coming back