<human>: I'm 18 along with my girlfriend. We've been dating for a year and a month. 

This weekend I made what is so far the biggest mistake of my life. A girl in one of the grades below me hit me up wanting to hang out at night. While we're hanging out, she starts scooting closer towards me until we're touching, then she started getting handsy. Being the awful person that I apparently am now, I went with it. Things started getting heated and I put a finger inside of her. Five minutes later I'm walking away telling her that I need to go home because I realized the awfulness of what I was doing. 

I never kissed her, I never put myself inside of her. I know that those things don't matter because I'm a dirty cheater. I have been crying myself to sleep every night since then and I feel so hollow and shitty. I don't know why I would violate her trust like this. She has told me over and over that cheating is something that's unforgivable. I fucked up so bad. This entire event has made me realize how much she means to me and how I don't want to be around anyone but her, but I know that if I tell her I won't be with her anymore. She deserves to know. I can't live with this guilt. Maybe I shouldn't live anymore.

Can you give me the gist of the text in a nutshell?
<bot>: went to second base with another girl and cheated on the girl of my dreams. I know that I need to tell her. I don't know what to do and how to handle this. I'm tearing up as I type this.