You are not alone in asking those questions. Living and loving anyone who has an addiction is incredibly difficult and puts you on a never ending emotional roller coaster. So the answer of how to react is all of the above. I know from sad experience with my partner who is an alcoholic in total denial.
As to it being an illness or choice I am currently in the camp of it being a choice. People don’t choose to be ill, i know as I watched my mum succumb to cancer and she would have given anything to have not had that awful disease. However, if you make life choices such as smoke, drink or do drugs knowing that they will impact your physical and mental health then that is your choice to make, and also your choice to stop and get help before the consequences become irreversible.
It comes down to the addict taking personal responsibility for their actions and asking for help. If they choose to do neither then no one else can help them.
The only person you can help is yourself. You have to focus on you and not their addiction. It’s hard to do, however it empowers you and breaks the cycle of enabling / codependecy which ultimately helps them.
Remember, if they fall don’t pick them up. They have to suffer the consequences of their actions to make them want to change.