How to get over fear of pain?

[RechargedSamsung]

I have a shotgun its in my parents room ik where it is and all yesterday since i was home alone i got their shotgun it was loaded aimed it under my jaw not really the chin and as i was sitting there with the shotgun under my jaw i was really thinking if i should do it or not but then i didnt because i got really scared of the pain i was gonna feel idk if there's any threads describing a 100% guaranteed painless and quick desth but that'd be nice if someone could drop a link to give more detail the shot gun my parents use is a okd one from the 50s i dont exactly know what it's called but i know it shoots a buckshot and it uses 12 gauge the gun is very vintage looking