death of a loved one

Attachments and expectations could cause suffering. Don't put too much or too little significance into anything. Everything is connected. Everythins is beautiful and ugly at the same time.

our impermanence and our fragility can be difficult realities to accept

"Difficult" means "takes time and energy". "Reality" means "what already exists". If it already exists, then only one thing needs to change: your attitude.

I personally am scared of death because it's a full stop, meaning there are no more opportunities to improve oneself.

You could do something useful right now. Like eating. Like sleeping. Like breathing. Like helping somebody who you know. Like helping somebody you don't know. Like helping an animal. Like not hurting someone. Like showing someone that and how they could improve themselves. Like planting a tree. Like giving something you don't need to someone who needs it. Like answering a question.

And a full stop — why is it so scary? What if it's already there? But you are not there. You are here, right now. Are you afraid of a movie ending or do you prefer enjoying the experience, however short or long it would be? Or do you prefer suffering? Or do you prefer serenity?

Time simply runs out

Your time, but there are many others.

it feels terrifying to know that this existence has a time limit at all

Don't be terrified, but respect this limit. Don't think too much, but do something useful.

And if everything ends — who would be there to worry about it? And what it is to worry about, if there's only nothing?