Should I go through with marrying a girl who smokes, after she told me of her intention to quit, knowing that I see this as a difficult task, or should I end this engagement?

If a Muslim is perplexed by a matter, the Sunnah is to seek guidance from Allah, the Mighty and Majestic. If it is good for him, He will guide him to it; otherwise, He will divert it from him and facilitate for him what is better.