Try a combination of Ibuprofen (Motrin IB) and Acetaminophen (Tylenol) or a mix of Aspirin and Tylenol. Ibuprofen, Aspirin and Naproxen (Aleve) are the same types of NSAIDs so don't take those together. Aspirin and Acetaminophen are used in the OTC migraine headache formulations if you want to try a combination pain reliever in fewer tablets. Bayer Headache doesn't add the caffeine that Excedrin Migraine does.
Not if you're having a Jack & coke, though!
Alcohol + tylenol = dangerous for your liver!
When you post, post into your hands. Especially when you feel like you're losing control of the trot. You are taking your hips, lifting them, and posting them up into your hands (forward to them, not on an angle or to the side). This will help you stay balanced as you post on a circle. It will also help you slow and even out your posting in spite of what the horse is doing.
It sounds like your horse may be getting away from you at the trot. You ride in an English saddle, correct? (posting in a western saddle is torture...) Make sure that when you stand in your stirrups, you can put four fingers between the pommel and your crotch, or else you'll have a very hard time getting your butt up out of the saddle! Now, what is your horse like at the trot? Head level, pushing forward and working... or head up in the air, hollow and trotting like a madman? If you can take some contact and get your horse soft in the bridle, and combine it with the "post into your hands" technique I mentioned, you can start to control that trot and make it post-able, more relaxed. I hope this makes sense! It's easier to teach in person. :)