SI is a real bitch. I think the body knows with hanging that it's a danger. plus, hanging involves a body envelope violation, which is something we want to avoid instinctually. I think that's why ODs and the like (SN, N, fent, etc) are much easier, since you're just drinking or swallowing something as opposed to physically harming your body. And yeah with full hanging, you definitely want nothing around you that you can reach once you fully suspend, cause your brain on autopilot is gonna be looking for a way out of danger.
I would advise against the CO2 method, though. If you know anything about how oxygen breathing animals work, you'll know that we have a well functioning system in our bodies for detecting CO2. If you hold your breath it will get uncomfortable, and then unbearable and you'll take a breath no matter what (even under water, letting water into your lungs). One of the most unethical ways we slaughter animals is gassing them with CO2, as they literally go insane and suffer immensely before death. So please do not do that to yourself, if you can avoid it. You'd likely have a better time fully hanging than this method.
I'm sorry you're going through this. I had many attempts where I was damn sure that it'd be my last day on Earth, but yet I'm still here. I wish you luck with whatever you end up doing.