<human>: Well, you mind find this childlike, but we met around 3 years ago, we fell in love, start a serious relationship, everything was fine.

One week ago, she starts talking with this older guy (3 years older) and seeks ''friendship'' and i was kinda jelous but it was okay, then i found out that he likes her and wanted to start a something, i tell her to please stay away from that guy but she continues talking and hanging out with him. 4 days ago we had a fight about it and confessed that she likes him too and thats why he liked to talk to him.

i had ask her about it time ago and she said he didnt like him and she was only seeking friendship, well know she is in a line with me and him. 

i lost my mind and go confront this guy, very strong and tall, and me short and skinny. He talks to me like he is trying to make her rid of me and such. 

i feel broken. she hide it from me, i know it's childlike but i don't know what to do, i have cry, stop eating, not sleeping. i'm very inlove with her.

Today she tells me she loves me and wont change anything for one like. i trust her. this guy keeps wanting to win her. 

I had girlfriends before but she doesnt and she tells she just wanted to feel someone else companion.... i couldnt handle it from then.

She tells me to change, (im very quiet and dont go out much, i tend to be over jelous with some guys) i have tried before, i promise her i will change. Now i dont know what to do.

I'll do everything for her, and i trust she will too.

Please, any advice for this first time teenager with his girlfriend?

Summarize the given text in a few sentences.
<bot>: GF find someone else, tries to go with him, i stop her, she tells me to change, i trust her and now im very sad about it, He stills fights for her.