I am a bit concerned about your plans to use a shotgun which you just purchased and have fired only a few times. Where did you go to test fire? Have you thoroughly field stripped and cleaned it so it will fire accurately and dependably and not jam or misfire? You ask if you should aim at your mouth or temple. Have you done your research? Could you reach the trigger or even hold it if you tried the temple? Shotguns can be very cumbersome and unwieldy. And a 16 gauge is pretty small shot. I am not sure that will kill you but if it doesn't, it will maim you pretty horribly.
I hear your suffering and my heart goes out to you, my young friend. You have tried an OD, hanging, knife to neck, and considered jumping in front of a train, It all seems so unplanned at that moment. May I suggest you take some time to consider all of your options?
Once you pull that trigger, there is no going back, no changing your mind. Your brains will be splattered all over before the recoil even happens. You won't even hear the bang.
You're so very young and deserve so much better!