How to cope with the very moment when death will come for you?

let’s say, I have one day, a week or a month left to live suffering from an illness,

A big problem with this line of thinking is that in real life you rarely know this, especially while remaining aware enough to commit to such a plan. It’s just a prognosis, based on averages.

or I know that after a while I’m going to have a heart attack that will definitely kill me.

1. You want to give up your remaining time because a heart attack is in your future? Usually you can reduce the chances with diet, exercise and medication.
2. For many heart attacks, survivability is directly related to how quickly you get medical attention.

Generalizing this back to what I believe is the goal of your question, it’s rarely certain or predictable. When it is, you may already be too far gone to make such choices (hence legal documents like DNR)

I get the point, reduce the suffering when you're at the moment. This is what I don't look forward to as well, not death itself, but dying in whatever form it takes. But know that any suffering is also finite, even the long ones, and hopefully you can avoid the few longer versions.

But for now, live for the moment. You get one shot at this, so don't spend it worrying about something that hasn't happened yet. Enjoy life, observe the details around you that we tend to block out as noise. Find ways to record and pass them on to others.

You don't ever know for sure. I've seen people with "less than a day left" take months, and people fully healthy drop dead from completely unknown health issues.

He's not a wizard. He is just a 'normal' human who didn't decay but died of an accident.

I like to remember what Tecumseh said

“So live your life that the fear of death can never enter your heart. Trouble no one about their religion; respect others in their view, and demand that they respect yours. Love your life, perfect your life, beautify all things in your life. Seek to make your life long and its purpose in the service of your people. Prepare a noble death song for the day when you go over the great divide. Always give a word or a sign of salute when meeting or passing a friend, even a stranger, when in a lonely place. Show respect to all people and grovel to none. When you arise in the morning give thanks for the food and for the joy of living. If you see no reason for giving thanks, the fault lies only in yourself. Abuse no one and no thing, for abuse turns the wise ones to fools and robs the spirit of its vision. When it comes your time to die, be not like those whose hearts are filled with the fear of death, so that when their time comes they weep and pray for a little more time to live their lives over again in a different way. Sing your death song and die like a hero going home.”

Thing is, you won't know. How could you?

I've been through it once and was at good health as doctors put it (not the trump like good health).

So live your life as you want to, it being over might be unexpected.

The leaving is no problem at all, but think about where you go afterwards.

"Right to die" is a medically assisted death that is available in certain places around the world. If you can't make peace any other way, moving to a place where this is an option may be the most helpful thing for you.

I guess most religions caution against getting too self-absorbed or materialistic in life. It's a common trap we fall into, and it's an awful place to find yourself when you're on your deathbed.

As I get older, I think more about the people who will be left behind once I'm gone. Will they be all right? Will my passing cause a lot of drama? If they still really need me, I will fight to stick around a little longer, even if it entails some personal suffering. It'll be over soon enough.

I guess if I'm in the middle of some personal quest, I may also fight to see it through before kicking the can? I dunno.

Otoh if my living on creates a greater burden, I might want to go sooner rather than later?

And I try not to concern myself with what happens to me personally after I'm gone. Whether you're religious or atheist, let's face it. Your fate is out of your hands at that point.

Same place you were before you were born. Why should it be anywhere else?

Do you live in a country or state where MAID (medical assistance in dying) is legal?

If you do, those laws allow for better than do-it-yourself options.

Death is the end

Do you judge a book on it last page, paragraph, or its sentence, or its last word, or its last punctuation, or the last page number at the very bottom right?

I certainly dont, i judge the book for the book it was, in totality (or as much of it as i remember).

Life is the journey, not the end. The end isnt the point, nor the goal, nor even the destination - its just a point no different than any of the others.

Release yourself from worrying about one moment, and allow yourself to see the beauty and capability of the entire life

Even in the absence of right-to-die laws, from what I've seen with older relatives, once the healthcare providers know what's what and divert you to hospice care, the drugs take care of the visible pain and, frankly, consciousness. These are generally practical, kind people who understand the odds and don't want to see suffering.

I found this book very comforting - Final Gifts: Understanding the Special Awareness, Needs and Communications of the Dying by Maggie Callanan.

I think the scariness comes from lack of familiarity. Getting a closer look at what it's actually like for dying people can help us face our eventual end.

The very moment itself is nothing to be afraid of. It's a moment. How many times have you fallen asleep in your life? How about passed out by other means?

I imagine it's going to be more like the latter than the former if you're still in your right mind when it happens, but both ultimately happen outside of conscious control. Consciousness shuts down and then it's all over.

As you rightly surmise, it's the indefinite period of time that leads up to that moment of no control that is the most difficult and is what requires the coping, if any.

For that, it's down to your own beliefs. If you're not sure what they are, you might want to think about who'd be hurt by your "leaving early", so to speak. (One of the main reasons I'm still here is that people I care about would be upset by it.)

And don't think that any decision you make now is set in stone. The day may come where your existence is unbearable. If you think you're there or about to be there very soon, that would make it an excellent time to seek some kind of help. Therapy. Pain relief. Emergency hotlines. Those sorts of things. (Been there. Done that. Will probably be there doing that again at some point.)

You may also want to take your religion, if any, into account, if your actions before the very moment are supposed to affect what, if anything, happens afterwards. (And if you don't know your religion, I can't help there.)