1. Once you've 'wired' your brain to depend on alcohol it's too easy to fall back into the habit of drinking. Think of it like someone who's given up smoking. They usually have to stop completely - having one now and then leads them back into the habit.
2. She may or may not want you back. Just because she's an addict doesn't make her a bad or dangerous person, but when she drinks the inhibitions might be suppressed enough for her to text you. She doesn't sound super obsessive to me though.
Congratulations on moving on and meeting someone new. It's a great reward for your strength of purpose.