How can I talk to my mother about this?

There is this concert/rave I really want to go to, and it's on the nineteenth of this month so I really need to get an answer from her, she has said it's an 80 percent no, but I think it's probably has changed after this weekend, when I did so many chores without being asked by her, my step father is very supportive, and I think if I ask and talk to her about it in person it will show her how bad I want it, I've been telling her via email. I am afraid of her blowing up on me for bringing it up in person, but I think it will also work better in person. I've always been better talking to her through letters or emails. I want to go so bad!! It's BHUNTER!! He rarely comes to America, this is a rare occion. What should I do? I haven't any attitude, haven't been fighting with my sis, been helping with my father who is very ill. Her reasons for why she doesn't want me to go are, she doesn't trust me (i used to do a lot of drugs, like coke, oxycontin, etc.) but i've showed a lot of maturity, been doing my chores, watching my attitude, keep my grades up, dance my heart out every night at the dance studio (ballroom). It's been almost a year since i've touched illegal drugs, and have always made it home on time, this thing i want to do will be overnight with my friends. also she doesn't want me to go because she needs someone to help with our grandfather, who had a stroke a few years ago, and I feel selfish that I want to go to this concert, and she can't do a lot, because her back is really bad, it hurts her to do the normal day stuff, but try helping a 160 pound 65 year old? that would put her in the hospital considering her doctors advise her not to lift more than twenty pounds. What should I do?