People often start at about 9mg of melatonin, and then work up as needed/tolerated. Some people here take amounts in the high 20mgs or even higher. Melatonin plus Benadryl is a strong dose of sleep-inducing medications, though, so take care if you are doing both.
Yes, GET THAT O2, ASAP!!!!!!! You're putting yourself through a lot right now that you won't need once you have the O2.