Of the list of terrible things humans struggle with, denial is right up there at the top. How the hell do you combat something you can't see? The worst part is how the sicker you are, the less you see. I remember drinking alcoholically for years, blacking out in bars and somehow getting home. Usually it involved driving. I figured that's just what everybody does. I assumed everyone woke up with vicious hangovers after every time they drank. Its just the price of doing business. I also assumed everyone drank alone as much as I did, which eventually was every night. Once people started making comments to me, i.e. "you should take it easy", or "take a couple weeks off", I had to assume there was something wrong with them. Addiction protects itself... brilliantly. When it senses a threat in the form of a parent, a spouse, a coworker, that person immediately becomes the enemy. When someone becomes the enemy of the substance that makes life bearable, I will shut that person out fast if they don't let it go. In denial your disease will show you every perfect piece of evidence to maintain control and masterly ignore anything that threatens your use. I remember concluding that taking prescription opiates 24 hours a day (without any physical pain) was a perfectly reasonable way to control anxiety. It was working for me! I felt great. I could do anything with confidence and ease. When I started getting sick, or lying for prescriptions, or stealing them from my friends and family, that suddenly became perfectly fine. I needed them, and if people understood how much I needed them they wouldn't care, at least if they were good friends. And if they weren't good friends all the more reason it was okay to steal from them. This all sounds so blatantly, ridiculously warped. When you are active in addiction it makes perfect, beautiful sense.
Perhaps this will sound quite disheartening to someone struggling with a loved one. All I think you can do is encourage people to change, remind them how much you love them, and hope they can see enough damage in their lives to want to change. Many times the more aggressively you confront someone, the more they entrench in their denial. They refuse to see any problem. I've seen better results with encouraging someone to look at their life and stepping back a little. If you give people some distance and set good boundaries with them without approving of what they are doing, it can make their denial a little harder to maintain. Just remind them gently that you think there is a problem and hope they will seek help, while acknowledging they have to want it. Perhaps the best advice I can give is to protect yourself and keep living, because god knows denial can last decades. It took me about 2 decades to wake up.
I hope and pray all of us wake up about all those things we hide from ourselves. Getting in your own way can take a lifetime to undo.