You have my sympathies. It is, as many others have pointed out, a lot harder to treat a hangover than it is to avoid getting one in the first place by trying to hardwire "water before sleep when drunk" into your brain. That said, we all make mistakes, and my usual solution when I get a REALLY bad hangover is:
1. Paracetamol (aka headache pills, I dunno what brand names you have but you can probably figure it out)
2. As much water as you can get without your belly retaliating (if you have some kind of commercial anti-hangover solvent tablets at home, have a dosage of those with the water)
3. Possibly some food, if you can get it down. Scrambled eggs are easy to eat and yum-full of protein.
4. Go right back to sleep, you'll probably drop dead as soon as the headache goes away
You'll waste away several hours of the day, and that really sucks. But when you wake up, you'll probably feel fairly well again, and imo that beats feeling like absolut crap all afternoon. Stay strong!