Broken trust with my significant other?

So I need a bit of advice.. I don't want to divulge too much information because I feel I would need to explain every little detail of my relationship with my girlfriend. We have been most mutually exclusive for the past 3 years (there were breaks) and recently we got back together again. I told her that I had smoked a cigar, even thought before she had asked me not to, and she was quite devastated that I had broken that promise and her trust with me. My question is what can I do to make her see that I am not that kind of person -- one that smokes or drinks or does anything like that. We have never really been apart and not talking for more than a day for these entire three years, even when we were taking a break, and I don't want to lose her...