Please, be very careful.
I preface this by saying that I firmly believe that suicide is not a moral evil. I in fact think that the world would be better if there was a good way to remove people like us. I think if governments actually cared about the happiness of their constituents, healthcare would involve the careful assisted death of people like ourselves.
With that said; I caution you because poorly thought out attempts can increase your pain, and even prolong it over time should you be unsuccessful.
I don't know what method is optimal. Unfortunately, research here is not really well tailored for our intentions either.
Nitrogen and helium gas asphyxiation, as seen with exit bags seem to be the most promising method, as far as I can tell. One must be careful that the gas is pure and that you have a regulator for the pressure.
"In the case of incompetent use of the equipment theoretically dedicated to such suicide, death may occur not because of the suffocation, which is the most common mechanism in such cases, but because of a pressure injury of the respiratory tract, resulting in rapid damage to the alveoli in the rupture mechanism, causing massive bleeding."
I strongly recommend carrying out proper research, and doing some private experimentation if possible before you try any method. For example, the paper I'm quoting, "Helium Suicide, a Rapid and Painless Asphyxia: Toxicological Findings" mentions that, although atypical, lack of use of a pressure reducing valve can cause the mentioned barotrauma.
As for experimentation, I can tell you from my experience that dehydration and starvation seemed reasonably easy to pull off. The only reason I ended up stopping that experiment attempt was because after a week, I was still working, and the symptoms began to interfere with my ability to work. And, since I wasn't committed to going through with it at the time, I called it a successful experiment, since I now know that it takes me a full week without food, and 3 days without water before I lose the ability to pretend to be fine.
Another thing. However much effort you put into researching methods like this; you should always put in at least as much effort to trying to fix things as well. Part of why this world is so broken and awful, is because it takes more effort to make and fix things, then it does to break them and make them worse.
I wish you the best of luck in life, whatever decision you come to, and in whatever might come afterwards.