It's always an amazement to me how people who have been smoking chemical soup cigarettes for years suddenly become "sensitive" to e-cig juices!!! THINK ABOUT IT.
Your body is finally able to gather and expel chemicals that are deposited throughout every tissue in your body, and you complain of a headache? Wow. Most people's "headaches" are usually just over-worked facial muscles, you are drawing too hard and too often.
There are over 4000 known used chemicals to treat inferior tobacco to give you a smoker's delight akin to a free-basing nicotine hit. And 24mg/ml of juice makes you sick?
Really, it's not likely you will ever get an overdose of nicotine unless you're drinking it straight out of the bottle. Find a density that keeps that Devil away, and never go higher than you need. And put the darn PV down once in awhile. RELAX, and trust what you're trying to do is right. It's been "Right" for thousands of people here.
VG and PG sensitivity? That's something that will present itself after a month or so of over-vaping. I did develop an aversion to PG myself, it's very unpleasant and does get worse cummulatively. Switch to VG, not quite so drying. DRINK WATER, Gator-aide, cranberry juices, green tea, increase your veggies and whole grains, use anti-oxident foods and drinks to help your poor body rid itself of poisons. AND DO GET SOME exercise. Walking and breathing with some vigor will help your body recover faster than you ever imagined. So get up out of that chair!!
Long, slow, easy draws from your PV will give you the best vapor production, and your nicotine. Your headache may be from trying to suck Carbon Dioxide out of a tailpipe (calm yourself down, this isn't going to hurt a bit!) Over-vaping with a higher Nic density will give you a headache. Just slow down your cadence a bit.
Give your body a couple months to come to terms with withdrawal symptoms. It will happen. The most important thing is to realize you are in a potentially much safer place, finally.....there are three ingredients to worry about, nicotine, VG or PG, and and a well performing PV. THIS IS YOUR NEW REALITY. The hand-to-mouth habit is the hardest part of all this to lose. But that will happen, also, just give yourself some time. Lately I've been forgetting where I put mine down. That's a GOOD thing!
Now the thing is to deal with dead batteries, dry or dying atomizers, and finding a suitable strength juice to keep your comfort zone. DO NOT PANIC, but do be prepared with replacement parts, a nice juice stash. It's not like you can run to a gas station for a fix quick. This vaping thing requires some planning and forethought. Cigarette smoking is a death sentence. Nicotine does not kill, smoke does. And it is no longer an option if an e-cig isn't working for you. So the only thing left is BIG PHARMA or COLD TURKEY. Your choice. Whichever choice you make will not be easy to pull off, unfortuately...it takes time, and you getting a grip on your over-use of the Panic Button.
Good luck to you!