See the resources compilation. As far as quick, maybe drowning or SN, but sadly a gun is the fastest way to die.
There's always the alternative of old age but I understand that resignation. I hate my life and who I am as a person but can't come to hang myself and can't get a gun.
I've lost most of my friends and have deteriorating health conditions that are my fault, but deep down know we all can make efforts to be a better person and fight for better circumstances.
Again, I want to die as well, but sometimes old age is the answer.
However, you could try an inert gas but that takes time to arrange. I tried and failed miserably with argon, so do your homework if you go this route.
But again, old age my friend. Don't make that mistake I did and try to bank on killiing yourself to get out of your problems. It won't work and you'll regret this decision when you still can't end your life years later. Live a good life and try to make amends for past wrongs.
Best of luck.