Loosing friends,what shall I do?

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This year before we moved to college,this girl was one of my best friends,we would always laugh together,hang out at lunch and talk about really funny things.We would always make fun of the guy who bullies us at college and just laugh it off.I feel like I am loosing one of my closest friend.Now we go to different colleges.Infact all of my friends go to the same college and I am the only one at different college.I have a quite bit friends but they don't understand me the way she did and others did. She is so different now,her looks have changed(that doesn't matter me a lot),she does weed,people told me she is almost high every single day,the most that affected me is that she is avoiding me she has got a boyfriend who does weed and is really hard core.I am so sad,i don't know why I care so much but I can't help but cry.Also,my other friends started smoking,I feel so alone.Worst part is the guy that used to bully me and that friends goes to that college I go to,he always makes fun of me,because I am obsessed.I want to know why I care so much,I need advice.I don't know what to do. I am not allowed to go with them as my parents are really strict,they are never online,the only time I talked to her was yesterday since september.I feel so incomplete without my friends.