I\'m the worst \"best friend\" ever..

Ok.. so.. there is this girl that I have been friends with since I started University last year. She was my first friend in this city, first person I met. I really care about the girl, honest I do. But I feel awful lately because I honestly can't stand her. She is literally the most annoying human being I know. Totally tactless, totally self-centred, extremely loud and obnoxious. She says the most inappropriate things regardless of social setting. As much as I don't particularly care about what the world around me thinks, she is actually embarassing to hang out with. Very rarely does anything embarass me. But... every time I introduce her to anyone new, or we hang out with new people.. I walk out feeling totally and completely embarassed because the truth is, people judge you on the people who you keep around you. She has no consideration for social setting whatsoever. I'm by no means wishing her to be fake.. I just wish she would pay some fucking attention once in a while. She doesn't ever seem to notice when a roomful of people are completely taken aback by the inappropriate shit she says. She has no consideration for what her friends would wish for her to say or not say in public. Basically.. she doesn't think before she speaks. And it drives me crazy. All last year I convinced myself that she would grow out of her annoying habits once she matured a bit and lost her highschool mentality. Didn't happen. Then I started with the hope that enough encouragement and moulding perhaps I could train her (for lack of a better term) in social tact. Hasn't happened. In fact.. she has gotten more annoying and embarassing over time. Every time I hang out with her I just want her to leave the whole time. I actually can't stand her. Problem is.. as soon as I get to the breaking point.. soon as I once again reach the conclusion that I cannot stand her and perhaps should stop hanging out with her so much, she'll have one of those days when she is actually really freaking fun to hang out with. Also... I know how much this girl depends on me for a sense of esteem. When I met her she had such low self-esteem.. she has grown a lot over the last two years in that respect.. but I fear that without me in her life, all of that growth will be lost. She tells me every fucking day that I am her best friend.. that wherever I go after University she'll go with me. She tells me all the time that she loves me more than her family. She practically worships me and the fact that I am her friend is what keeps her going in her life. She thinks I am the smartest person in the whole world.. she puts me on such a pedestal I don't know how the get off of it!! Ahhhhhhhhhhhh she just DRIVES ME FREAKING NUTS!! And I know that anyone who reads this will probably tell me that if I can't stand her I should just stop being her friend. It's not that simple. Like I said - first friend in this city.. relies on me completely.. don't even want to entertain any of the possibilities of what she might do without me in her life..... and the simple truth of the matter is that I really do care about her. Not sure why.. but I do. I cannot pinpoint what it is that makes me care so fucking much about her.. but I do. I feel like her well being is my responsibility. I just want her to grow up so bad.. she is 2 years younger than me in the first place, but on top of it she is totally immature even for her age. I'm 21.. and it feels like I'm spending my days hanging out with a 16 year old.  buhhhhhhhhhhh