How do you cope with unexpected triggers that remind you of smoking, even when you’re committed to being smoke-free?

I often find myself wanting to self harm, to give myself an actual solid reason to cry about, by ruining the beloved child of my parents. Of course I wouldnt do such a thing, so why do I want it so bad?
Why do ex-smokers still dream about smoking, and how can you deal with those feelings of failure when you wake up?