What is something you can’t have just one of?

If you're serious about it, the first thing to do is change your rhetoric.
Don’t say "I need to quit drinking" or "I should quit smoking" or "I'd love to lose weight".
Instead say "I am quitting".
When you use words like "need to" or "should" or "ought to", you set your mind to think that the conditions required for you to quit aren't met yet, that there is some extra step for you to take before you can quit. When in actuality there isn't anything stopping you but yourself.
Giving up is hard sometimes, but it is a lot easier when you genuinely set your mind to it and to do so adopting the right rhetoric is a fundamental key for it.
Haven't had a drink in years, one of the factors is because I genuinely put my mindset in imperative mode rather than using conditional.