.50 Cal shot to the head does the trick.
Well usually a shot to the head is the fastest.....I don't know if it hurts or not tho but since it happens so fast I dout you'll feel it......but please don't die. You have so much to live for.
Hold your breath for 5 minutes
If you really want to kill yourself, than it shouldn't matter how quickly or painless it is. So if you are afraid of the pain along the way, then you are truely too afraid to really kill yourself. So that should tell you not to do it.