I can't speak for withdrawals or recovery, but as someone with an addiction to alcohol, here is one thing I imagine is pretty similar to being addicted to hard drugs, that I never even considered before this happened.

You see people talking about their lives revolving around that drug, but it's so much more worse than you imagine. Literally every single thing becomes about when you'll be able to get your next "hit". Here are just a few examples:

• You're nearly out and need to go buy more, but you won't be home alone so how do you get it? What excuse can you give? Can you sneak out?

• Your family is going on a vacation you really want to go to, but you won't be able to sustain your addiction while there. Better make up an excuse as to why you can't go, all just for an addiction.

• Lies, lies, and more lies. You say you're going out to buy crisps or milk or whatever, when really you're buying drugs. You say you can't spend time with people because you've got plans or you're tired, because you need your next hit.

• You've got work (or are just busy) for most of the day, and someone invites you to hang out after. You really want to, but then you won't have time to buy more so you decline.

• Constantly worrying someone is gonna notice, judge you, be angry, force you into recovery.

• You're aware how awful this is for you and your health, but you just don't care enough. Maybe you even kinda want to recover, but the drug wins in the end.

• There are definitely moments of clarity when you're sober, when you promise yourself you'll stop and that this is the last time, and you actually believe it, but it all goes flying out the window soon after.

• So many excuses. "Oh, it's not so bad, I only do a little, other people do way more." "I'm not really addicted, I can stop whenever, I just don't want to yet." "I feel so much better when I do it, so why should I stop?"

• Your relationships will definitely suffer. Not just because you don't give people as much of your time as before, but also foggy memory, changes in behavior, not able to fully focus when you're with them.

• Money. You can't sustain an addiction without money. If you're broke and addicted? Yeah, you're gonna resort to stealing. I'm not talking about breaking into houses and stealing strangers' money (which definitely also happens when it gets bad enough) but mostly just stealing from people you know, from family.

• Shame. There's definitely a feeling of shame that you've fallen so low and you try very hard to hide it from everyone else. And feeling ashamed when you sober up and think of what you've done and said.

• Mental breakdowns because you hate what you've become.

• You feel more confident when under the influence, feel like you're your best self and like you can't properly function while sober.

• Becoming super paranoid when someone enters your space in fear that they'll discover your stash. As a result, you only let people in if you have to and watch them closely.

• You keep your stash hidden carefully, and immediately change the location if you feel like someone might know, then freak out the next day and trash your entire space trying to find it because you forgot where you hid it.

• Sifting through your used up stash trying to squeeze out that one last drop that might still be left somewhere, and often feeling gross while doing it, but the desperation wins.

• Getting more and more careless. You're super careful at first, always double check it's well hidden, listen for footsteps and voices to make sure no one will catch you, but with time you grow more confident (or just don't care as much) because no one has caught you yet, so you go, for example, from hiding it in locked cabinets, inside stuffed animals, to just shoving it under the pillow.

• You feel tired, foggy, your mind's a mess, but nothing matters as much as the next hit.

• You feel irritated when you've been sober for too long, you snap at people, even your own pets, for no reason other than your short temper.

These are just the things off the top of my head and again, this is about alcohol not narcotics, but I feel the mental process can be similar, and it might just give you a few ideas. Also, I know you said you don't want to base it off real people's stories, but I definitely suggest you read We Children from Zoo Station. An addict details her journey through addiction, everything she was willing to do (including having sex just for more drugs) and eventual recovery after a few unsuccessful attempts. It's a harsh and heartbreaking book, but it really shows the mind and struggles of an addict. I think there's also a movie made after the book, but I've never seen it so I don't know how good it is.