Over the counter drugs would include:
Diphenhydramine (Nytol) - this will just make you tired and in high doses, delirious
Dextromethorphan/DXM - this will make you feel sick, racing heart, but does have fairly strong painkilling effects. Doses above 400mg are generally very pukey and unpleasant
Some form of codeine - proper cold water extraction procedure could get enough out of a few boxes to give a dose which is strong opioid effects and painkiller, also lots of nausea and itching
Metoclopramide - may help nausea but will cause apathy and sleepiness, but not relaxation
The only thing available legally which has the kind of effects you're looking for is alcohol. Lots of it. But even that will not dull your senses enough to not notice that you're hanging by your neck and can't breathe. It will lower your inhibitions, may make you unconscious if you manage to consume a large amount without vomiting. But few things will make you sleep through a hanging, not even prescribed sleeping pills.