My girlfriend seems like she wants nothing to do with me and keeps telling me she doesn't know if she wants to be together anymore. One of my problems is drinking. I told her I would fully quit and be there for her whenever she needed me. And I was telling the 100% truth. I want to marry this girl but now she's saying her ex coke head jailbait boyfriend is straightening out and she wants to give him a chance again. How do I prove I can be the perfect one for her? Because I am...

You need to stop pushing the subject of remaining together while she is telling you what she expects of you and you are ignoring it. You want her to stay, fix the problem, mainly drinking, don't tell her you are willing to do it, SHOW her. However, I would personally leave this be, she obviously wants to be with her EX instead, what is the point of doing something for someone who is not even interested in being with you anymore? Seems like her mind is made up, break it off completely, change the drinking problem for your own benefit, and move on.

Of all the things in this world we have control over feelings is not one of them.
You cannot control her feelings or wishes, but you can control your drinking.
You also cannot SAY you are going to do something and expect people to believe it without putting the effort behind it.
Since it sounds like she is bound to give her ex another shot, I'd work on myself.
Get the drinking under control for your sake, I think you will find that most women just don't like it anyway.