Fear is normal. It doesn't mean "your time hasn't come" or "you're just not ready". All that implies is that you are not in control of your fate, or that you need to suffer more by staying alive longer. I don't agree with either of those sentiments, but it is generally good to think things through with consideration when, or if, you can.
The action is simple: "just relax and pull the trigger." But simple does not mean easy. It's usually the simple things in life that are the hardest to do. Don't be hard on yourself about this, it's hard enough already. This is why a lot of people take a substance to help deal with the apprehension.
It's your choice, and unfortunately, we all must die alone. No one can pull it for you, or walk with you into death, as you're well aware. Depending on the caliber of ammunition though, as long as the projectile is a decent size, the likelihood you'll taste anything is very low. Bullets travel faster than sound. You can only speculate, and your speculations are probably more frightening than the reality.