Well, yes, most addicts lack self-discipline in one way or another. And yes, addiction is complicated, but why you started to smoke weed every day ? Just smoking weed on the weekends gave you so strong cravings that you couldn't stop ? Maybe quitting amphetamine is easier because you feel how destructive they are, so you got strong motivation to stop, at least this how was for me. With weed people think smoking is completely safe and start doing this every day, and then of course after some months of smoking so much you get addicted. But why in the first place smoke so much ? I think it's similar to video games and porn, if you start doing this very often then it's not because these things are very addictive on their own, but because of your own problems of some kind, don't you agree ? On the other hand addiction to speed is "pure", kinda similar to addiction to nicotine, you feel very strong cravings for many months after quitting, most people don't have something like that from smoking weed on weekends, with hard drugs this is the norm.