Hey there, Death effects everyone even if the person who died wasn't someone who was necessarily close to us, knowing that someone who was even just a small part in our lives has passed away can be a terrible experience, it makes us think so much about the people around us and our own lives. It's normal that you feel sad even though you hardly knew this girl.
A lot of the time death is unfair on people, young people die who did have so much ahead of them and it can make us sad. But, celebrate their life, don't dwell on their death, I'm sure they wouldn't want people to be sad.
However, take this time to re-evaluate your life and think about how you could start living your life now, put your effort into starting to make your life the way you want it to be, have no regrets, make your own mistakes but most of all, have fun because that is the most important of all.
Think about the other people in your life, cherish the time you have with them, don't let any second go to waste. Take all the chances you can and just enjoy yourself. That way, if anything bad happens, at least you can say you made the most of what you had with this other person, you can truly celebrate their life.
Right now, wishing you could be their friend, wishing you could go back in time, none of this is going to change, act on what can be acted on, see what could be, not what could've been.
I hope things get easier soon.