How to deal with family members who neglects their health?

I'm really desperate to what to do about my *family member*.
She neglects her health, refuses to get treatment for a very painful but otherwise benign disease she has.
She agrees that she needs to take certain medication and supplements (one being highly suggested by doctors, like Folate), but just doesn't take them. Every time I ask she says "yes I'll take it" but "later". And later means never since about 10 years. She even talks at length about the possible benefits etc.! BUT doesn't take. Generally I'll take it "later" or "not now" means "never ever" in her vocabulary. Also when talking to me about things I want to do (personal things). like "not now" means you can't do it ever.
She has a very bad cough every morning (she's a smoker), doctor prescribed an x-ray or rather CT of the chest. she didn't do it. Also we have insurance so that's not a financial thing. It would be practically free.

Once in a fight I confronted her with all this neglect and she said she just want it to be over.
I used to try to kind of encourage her to get help or treatment etc., but she always gets mad at me for this. Like even trying talking about getting an appointment for the painful condition by the doctor the GP recommended to her for the disease!, after she showing me the referral etc. (the GP was begging her to get treated too). Why does she show me the referral if she doesn't want to go there, EVER?

I have another family member who is exactly the same, and they both hate me like hell for all of this attempts. Like the other family member had some mental health crisis a couple of times and I helped with finding doctors or getting her to ER appointment etc. She now accuses me of ruining her whole life by this, which I do not understand, as she was perfectly agreeing with going to the doctors back then, and it was not really my decision in the first place as other family members also wanted me to find her doctors etc.

The point for asking this is actually not helping them. I want to know how I can live with it without getting too triggered and miserable by having to witness this.
Obviously me trying to "help" causes them more suffering.
And me watching them not caring for their health, and also drinking sort of a lot hurts me.

I primarily want to solve the issue for myself. Done with trying to help basically....
One thing I would really want to find out would be: why exactly would someone even do / be like this....??!? It doesn't make any sense to me and I genuinely want to know, from a place of compassion and not judgment....

Coming from experience, the only thing you can do is distance yourself from their lives and focus on yourself. Only you can make you happy, and the same can be said for them. Only they can make themselves happy. You can't save someone who doesn't want to be saved.

As of why people do this, I do not know. It is hard to watch, the solutions are simple, but I guess some people just like being miserable.

You aren't their parent tho, and its their choice to live their life the way they want. So you will just have to learn to focus on other things. Use this to motivate yourself to be as healthy and independent as possible. Being an example, with a life well lived, is usually the best medicine. It's all you can do really.

They will respect you a lot more for it too, and in turn, what you are doing is showing respect for their choices. From a healthy distance of course.

Don't sacrafice yourself or your mental health for anyone, you are just as important as everyone else.