One simple trick I learnt is to change the way you talk about the thing you want to stop. Instead of saying "I'm trying to stop eating 12 donuts at a time", you say "I don't eat donuts". It's a strange psychological trick that seems to work well.

Or "I always take the stairs" instead of "I'm trying to take the stairs".

You're right, that if you can't stop doing something, it's because you don't really want to. There's a big enough part of you that doesn't want to change. If you really, truly want to change something about yourself, then you do.