Has my liver been significantly damaged by my drinking (I am 21 and I drink a gl of water every 2 drinks)?

No, I think your good to go. Your still young, and as you get older you may tend to drink more to catch that buzz. I think it would be a good idea to quit now while your still young, and if you do quit you will have nothing to worry about as far as your health goes in the future. Maybe just cut your drinking down to 3 or 4 times a week or less. Have fun while your young.