I sympathyse with you, it's so tough to be different in any way when you're a teenager. I had a liver transplant earlier this year down to possible AIH (don't worry though, it's very unlikely this will happen to you AS LONG AS YOU LOOK AFTER YOURSELF!! Most people with AIH manage it with medication, and transplants only happen in a few cases - they're not even sure if mine was AIH)
I'm 25, young free and single, so although I don't have the same temptations as a 16 year old, I know how hard it is to have to think about your health all the time, when all your freinds don't give it a second thought. But thinking that you don't mind having a couple of years knocked off your life as long as it's a good life is the way a lot of people think when they are young, but unfortunately life just isn't like that. You don't just have a fantastic life then suddenly die; you get ill, very slowly and painfully and your family watch you suffer and fight for life, you suddenly have more to worry about than weight gain and chubby cheeks (though i understand how annoying that is!) and finally you may end up on the transplant list, and if you're fortunate to receive one, then you have a long way to go to recover and then really have to watch your health for the rest of your life. I know this sounds harsh, and I really don't mean to scare you. I totally know how you're feeling - it's just so unfair that you have to deal with this at your age, along with all the usual teenage stuff, but this is the reality you're faced with. However, if you do watch your health and take those horrible pills, you can live a very normal life, with just a few adjustments. I hate the predinsolone -I'm 25, but when on steroids people think I'm 17 cos of the chubby cheeks! But it' a neccessary evil I suppose.
Every drug you put in your body -from paracetamol to alcohol- goes through your liver, even if you puke it up an hour later the alcohol is already in your blood stream. I suppose one way to deal with not drinking when your freinds are is to find other things to do - I know this doesn't sound like as much fun as partying with your freinds, but you could try taking up sports (if you haven't already, and if you're well enough that is) which would also help with the weight gain. If you're focusing on something else, something healthy, sometimes it really can take your mind off those tempting unhealthy things.
I don't mean to sound preachy, cos sometimes I don't follow any of my own advice and I'll be honest with you - I still drink in moderation (I do have a healthy liver though, it's more my general health I need to watch) I've tried to find a balance between living my life the way I want to live it, and living it the way I 'should' live it, and I have found it very difficult - it's something I never had to think about before. I just hope you don't go into the 'live fast and die young' way of thinking, because I know some people get away with it, so it's tempting to think you will too, but you won't get away with it for very long.
I wish you the best of luck! I hope you find a way to deal with all of this, it's a lot to cope with at your age. But being a little different from everyone else will teach you to be strong - though that probably isn't much consolation to you now!!