How do I become less insecure about my girlfriend?

She went to eddies which is a nightclub with her older sister, this worried me because I don't think I trust her. She's only 16 but has had 3 sexual partners from the age of 14 I hate this because it shows how easily she gives up sex, she seems very naive about guys because she has been treated badly before and used for sex. My previous girlfriend cheated on me she was also what I consider a slag 10+ sexual partners at the age of 17. Everytime she goes out in the night time I imagine she's talking and flirting with other guys, she texts me after the night saying it wasnt very good, but to me it seems like she's lying because why stay out till 2 in the morning if it's rubbish? We had a huge argument before cause she wanted to go to a festival where she slept with 2 strangers and I didn't want her to go because of it, she's no longer going because of my insecurity. I have never cheated on a partner, I'm 19 years old we've been together for 6 months, also her sister has recently broken up with her boyfriend and is going through a slag stage and sleeping with a number of new men, I was wondering if anybody knows why I feel like this, and how can I help to make it stop because I feel it will inevitably break us up.�