I need some relationship advice?

Your question can be answered by another question. Did you ever notice her behavior like what you are saying she goes out alone to get some drinking binge? Have you learn it from those around her other friends that she does go out to drink? If your answer to these questions are yes, then, it is not your fault to feel this way about her. You can either accept the way she is now or take a risk of discussing to her about it in an honest way as to ask her if she is drinking too at her own home. Alcohol is a habituating drug. One can be hooked up if one is not careful to hold on to its own self control. Just because you yourself is not in the acceptable age to drink, you cannot give a decent intelligent talk about the harmful effects of alcohol. Your way to make a clear point is to approach to her in a logical way in that you care for her health and well-being. this may work out for her to stop such drinking alone. Try to understand what makes her do such habit of drinking. Can she be in a stressful situation and is she expressing her anger and frustration to something? Hope this gives you a little idea why such habit of your g-friend came about.

are you scared that she going to cheat? because when i go out or my boyfriend does without each other, we always end up drunk texting each other :) and it removes doubt. maybe send her a few messages to see how she doing but not constantly, it might put your mind at rest :) i take it you live in the states where drinking age is 21? she just enjoying going out,and hey you'll be out in 2 years joining her :)