Feeling betrayed By Female Best friend but It's more complex

I (20M) has a female friend(20F) and i also call her that, she gives me all of her attention to me I am her boyfriend but actually has a boyfriend at her home (we're in college in same class in another city and she has a boyfriend back at her home), initially she told me that she dosen't like physical intimacy but i peeked at her phone one day and i found several kissing and intimate touching videos with her boyfriend on her mobile phone.

I was heart broken (high BP for hours and sudden panic attacks for several days) when i saw those photos because i was getting attached to this girl
I haven't told her that i know about that photos and whenever i ask her about such things she says this things scare her.
I'm not forcing her with anything and i didn't expect anything but i thought that this person I've Been talking to for several months was different that others and at the end of well all i found is my disappointment.
I was also sad because our relationship as Best Friends was built on sharing everything with each other without any judgements.