Wow this is interesting. You could give yourself a mega dose of insulin prior to completely knock You out which is available without a prescription at Walmart just ask for novolin n. And once you've got an IV line set up and you're all tied down and strapped to avoid messing up it would be an absolute painless death precipitated by hypoglycemic coma. The previous posters paper however indicated that death within seconds is possible in large primates in super saturated concentrations with high lethality, at 130mg/ml for a large primate such as a horse, the human LD50 should be about half if not less.
What sucks is I'm a needlephobe, although if I can booze myself out enough and bite the needle I could probably work up the courage.
As far as the IV line sting, an emulsion of DMSO & benzocaine applied topically should be enough to numb at least some pain as it's a transdermal carrier and benzocaine is not illegal and a powerful topical anesthetic that's also cardiotoxic. taken in conjunction with a third lethal RC opioid dose like Isonitoetazine which is the most fatal among all putting fentanyl to shame and readily available on the darknet for cheap the chances of anything going wrong are slim to none.
The greater combination of methods and independent formulations the greater the likelihood of a rapid death without fail or at worst being found only to be rescued with substantially life-changing injuries. Self-hypnosis audio recorded over many sessions rehearsing the protocol at a quasi-conscious suggestive state would also mitigate the SI and upon A trigger event set or when the time comes make the process most effective.