What's more difficult? Knowing beforehand the exact moment you would talk to a person for the last time, or staying oblivious to the odds of future.
In both the scenarios, though the event is common, the before & after is very different.

In the 1st scenario, you know the exact moment a person will stop talking to you or the exact moment you’ll stop talking to the person. They won't be part of your life anymore.
In the 2nd scenario, you are not aware that one day the person will be gone. You both live your life as if it will last forever.

Which scenario will you prefer?

Some people say when you know that something bad is going to happen, it's easier to accept, that you can prepare yourself. But to me that doesn't sound ideal. Here you know the exact moment a person is going to go away from your life, so every day that you see them it's going to remind you of what's coming. So all those moments which you do get to spend with them would be a sad reminder of that sad imminent future. It's only going to remind you that, that face & that voice won't be with you anymore. So will you be able to enjoy whatever time you have left with them when all you do is worry about the future?

In the 2nd scenario, you don't prepare for it. Because you were not aware that it's going to happen. So you enjoy all the moments spend with that person. You can live in the present, instead of worrying about the future.

Now once that moment has happened, and that person has left. What now?

In the first scenario, maybe you were prepared for it. You might not have any regrets. Nothing left to say or share with them anymore. You had your time to say last goodbyes. And now maybe you can focus on moving on with your life.

While in the 2nd scenario, all you have left now is regrets. You wish you could say this, you wish you could say that. You wish you could hug them for the last time. You struggle to get closure but it will seem impossible. Every day you wish you had more time with them, to say goodbyes, to say it all. But now those goodbyes are left with you, all those you wanted to say, they keep you awake at night. And it's very difficult to accept the loss.

To summarise none of these scenarios seem better, you will be miserable in both. But I think I will prefer the second scenario, I will have a lot of regrets later, but at least whatever time I get with that person, I can spend it without worrying about a future.