It's how addiction works.
It's neurological.
The dopamine and serotonin receptors react to the addictive substance. Whether alcohol, drugs, gambling or whatever.
Every person who is 'addicted' to a substance will experience a 'high' when they use that substance. Thing is, as you become addicted to the substance when it's no longer a high but just the relief of the craving of the substance being fed so its not a high as most of us would think, but a general feeling of relief which appears to feel good which it's difficult to replicate with other substances or behaviours because it doesn't subdue the craving in the same way because the brain is used to responding to that substance.
So for a lot of addicts, it is completely irrelevant what joy they can experience in other areas of their lives. Their dopamine receptors are still telling them they need 'the substance'.
Without that substance that the brain is used to and craving, it does not matter how fantastic and beautiful real life is and how you can lose it all by being a twat and drinking or using drugs. Any addict knows that and still loves all the beautiful things in their lives.
But there's a gnawing sense of insecurity, unease, emptiness and a physical feeling which is so uncomfortable and intolerable you want to crawl out of your skin. And using does make that better if just for minutes/hours and after that, you f* everything up again.
No matter how wonderful you and your children are and how truly joyful that experience is for her. It doesn't scratch that itch of that empty feeling.
Addiction is that the person is addicted to that substance. Doesn't matter how shit everyone else thinks it is. That's what they're addicted to, it's what their brain is responding to.
How many talented, rich, successful celebrities with access to the most expensive treatments and lots of friends and family die of addiction?
Is that a choice that brilliant, successful, talented, rich celebrities make the 'choice' to die alone OD-ing on their substance instead of living lives free of addiction?
Addiction damages the brain. Almost all studies evidence that when you're an addict, you get a flood dopamine using the substance and then your dopamine levels go to below the baseline of a 'normal' brain so you feel more anxious and low in mood than if you hadn't used a substance. So you need more to try and get 'normal' levels in the brain.
You should walk away. But don't think it's a choice that was made when it wasn't.