The most painless way to commit suicide that ive been able to come up with is suicide by morphine overdose. If you get morphine in its powder form and mix a considerable amount with water and then drink it, you will overdose. But this overdose isnt going to be like a coke overdose (heart beating so fast it feels like its gonna blow up) instead you will start to feel weak, fatigued, relaxed, tired. Then you will go to sleep and never wake up. This happens (from what i understand) because morphine is a depressant and it works by slowing down your heart rate, so if you consume too much it will slow down your heart rate until it stops. This was my #1 plan for suicide for a long time and it still kinda is, but the only caveat is how hard it is in my area to find pure morphine in its powder form.