Why does my brain need to finish the whole amount before I can relax?

For example, if I buy a gram of cocaine, I feel like I can’t sleep or properly relax until I’ve finished it. What’s strange is that the amount doesn’t seem to matter — if I buy half a gram or 0.3g, I get the same feeling of relief once it’s gone.
I can go months without using anything, but whenever I do buy it, this pattern seems to happen again. I experience something similar with lean. I also have ADHD.
Why does my brain seem to work this way? Is this related to addiction, ADHD, impulsivity, or something else?