Hey buddy, a lot of what you describe reminds me of my more chronic symptoms when I was using mephedrone daily to work, and after stopping taking it, particularly for a few months afterwards. Increased anxiety is a frequently-mentioned side-effect of withdrawing from stimulant medication, and there are a range of other effects, but the general consensus I think I'm seeing from my reading around stimulant abuse seems to be that, just like most other common types of stimulant drugs, the changes you notice when you withdraw are a symptom of your body's having adapted to the drug, and given time without the drug and a reasonably healthy regime (more sleep, fewer McDonald's ;o), your body will adjust back to its baseline position.
Psychologically, things may be a little more difficult for you now that you have labels to attach to the states you find yourself in after withdrawing; the HPPD, tinnitus, anxiety and respiratory symptoms will probably be a significant cause for concern, particularly if you don't remember them occurring before your abuse of mephedrone at the level you do now.
If you would be prepared to seek help, and if you haven't already (and apologies if you have mentioned this previously, but I couldn't find mention of it), I would be inclined to recommend you research drug counselling and rehabilitation services with a view to speaking to a medical professional - in confidence, if that would make you happier - who has experience with the effects of stimulant drug addiction such as cocaine and crack cocaine, methamphetamine, amphetamines, etc.
One of the things I gather from your post you're already aware of is that mephedrone is not a widely-known substance of abuse in the same way that cocaine, amphetamines, and methamphetamine are known in America, but enough is known about mephedrone's effects on the body to be able to say that it is similar enough in action to these more-common drugs that treatment approaches for those drugs - cocaine, meth, etc - may benefit mephedrone users, too.
Lastly, I think it's worth mentioning that I spent a lot of time convincing myself that I was about to suffer a horrible and untimely death almost every time I took mephedrone towards the end of my period abusing the drug. Part of this was probably increasingly-poor quality synths, but I don't for a second deny that another part of the anxiety and low pain threshold must have been my body adapting to the drug, which left me anxious, tired and uninterested when I wasn't dosed-up. I started to experience very real chest pains; lying in bed at night trying to sleep after a week of taking 1g per day, I would experience stabbing and "snatching" pains in my chest, and I couldn't lie on my right or left sides for several months after discontinuation due to these pains.
During the day, both while using the drug and for a short while after discontinuation, I would experience similar discomfort, and would have to sit slouched to the right in my chair at work to try and stop the apparently-uncontrollable chest pains. Eurgh. I honestly though "I'm going to have a heart attack, any minute now...". Not the nicest way to spend your days, is it?
Having been clean since the drug was scheduled in the UK, in April 2010, I'm feeling so much better it's unreal. I've got myself to the gym and lost two stone (although I took things really steady for the first couple of months at the gym), I've found myself a much better job after realising the only reason I put up with my old job was the drugs which made it less-unbearable, and I've sorted my life out. I may also have left my girlfriend, but that was as much because I didn't want to wreck her life the same way I could see I was wrecking my own as for the fact she was batshit-crazy and thought Hello Kitty clothing should be worn as the uniform of the British Royal Artillery*.
Anyway, I digress. I sincerely hope you have some luck answering your worries about the changes this drug has caused to your body, and I also hope you find answers from people who you respect enough to wholeheartedly trust. If I've learnt anything about this experience, it's that 90% of my battle has been against myself; only 10% of my brain seems to have been giving rational arguments - the rest has been assuming the worst with only a shred, if any, of evidence to support it.
All the best. You'll be fine. Honest injuns =o)
/dm
* May not be true. May have been made up to cover up the fact my ex g/f was pretty alright...
