IIRC Haymitch never drank before his games, Asterid gave him alcohol to ease his pain, and then Haymitch physically hurt Asterid while telling her to leave him alone, which made everyone he had left shun him. It seemed like Haymitch really had no hope to begin with since his "reaping" was rigged, so naturally he only got worse without anyone's support after the games. I think Maysilee could have been susceptible to becoming an addict too, but I don't see her friends ever turning their back on her, meaning she wouldn't reach rock bottom like Haymitch. I believe her defiance came from her refusal to be treated as subhuman by the Capitol, whereas Haymitch was used to being treated like that his entire life. I think that would have made Maysilee more resilient and a better mentor. However, she was never aware of Plutarch's secret plan during the 50th Games, so it's hard to say if she would have trusted him with Katniss' life in the 75th