I've got just over 6 weeks clean after a 3-year heroin habit. But I've been trying to quit for over a year now. The only thing that's let me even get 6 weeks is my recent decision to get on suboxone to control my cravings. the cravings are the absolute killer for me in terms of failing to stay quit. And I've thought a lot about how to describe heroin craving as I experience it. Here are a few blurbs.
Metaphorically speaking, the experience is like knowing that my best friend's incredibly gorgeous and sexually voracious wife absolutely wants to fuck my brains out. All I have to do is climb into bed, getting under the heavy down comforter she's waiting under. I know in every conceivable way that doing this is a *terrible* idea...a terrible thing to do, period. The closer I get to doing it, the worse I feel about myself. But eventually, I tell myself, "aw fuck it, you're a shitty person anyway...prove everyone right." and then it's game over.
Another metaphor, this time for trying to banish a craving from my thoughts. Imagine you get a phone call. It's a friend of a friend...someone you know isn't a total nut, but you don't know much more than that about him. He says, look, I know this is crazy but I *need* cash right away (say for surgery or something). Our mutual friend says you like fast cars. Well, I have this BMW M3 that's got 500 miles on it; nearly new. I'll sell it to you for $1000 if you can give me the money within an hour. ... Trying to get past a craving is like trying not to think about that phone call for the next hour. I know it's probably a scam. There's almost 0% probability that I'm gonna actually buy the BMW. But just try not thinking about it. The worse the craving is, the more valuable the item for sale is (a guitar, a car, a house, etc...) and the shorter the putative waiting period, which increases urgency.
Those are both metaphors, but I think they conjure the experience not too badly, at least as I have it.
More literally, if a strong craving is coming on, I usually get very very depressed. I know that fixing a good shot of dope will make the depression abate. The longer I hold off, the more depressed I get. It's as simple as that. The metaphors I used above just mimic the head games I play to overcome what little judgment I have during the depression.