Why are airplanes described as the safest method of transportation?
Good news

The good news is: you really are safe in an airliner, so safe that you could spend every minute of every day of the the rest of your life flying in one, and still not face any greater meaningful risk of a disaster.

It's difficult to answer your question why it's so safe (i.e. what makes it safe), but the record is there, plainly, that it is so safe.

Bad news

The bad news is: if one has an irrational fear of flying, then no objective facts or understanding will help lessen it.

If the fear is based on misunderstanding or lack of knowledge, then of course learning more will help overcome it. However, that's not the basis of a phobia, or the kind of fear you seem to be describing.

Advice to understand flight and aircraft is well-meaning, but not actually that useful. Someone can have an excellent understanding of aviation 
engineering and aerodynamics and all manner of subjects, and know that it is exceptionally safe, and still suffer from crippling fear of actually flying.

Everyone is different, but such fear is often brought on by an event or change in life that has nothing whatsoever to do with flying, but somehow gets expressed in a fear of flying.

The comfort and support of another person on board could be the most valuable thing you seek before your flight. I'd suggest expressing your fear, without embarrassment or shame, to the staff at the gate. They and the crew on board will have helped people in such cases many times before, and they are good at it.

Simply getting used to an object of fear is one of the best ways of dealing with it.

I hope you make your flight, and I hope to hear that having done it once, the fear diminished by being stared down, and that subsequent flights render it lesser still. Please tell us about it here.