Overtreating means getting more treatment than you need or continuing to treat with the same healthcare professional for months even though you have been feeling for a while that the type of treatment you are receiving is ineffective. If you have been seeing a chiropractor for over four months, then you should at the very least ask him when he believes your treatment should be over and when you are expected to achieve maximum improvement. Generally, seeing a chiropractor for longer than six months will likely make both the insurance company (during settlement negotiations) and the jury (if your injury case goes to trial), look at your injuries with a greater skepticism. They will be asking why you didn't choose a more "serious" treatment, such as medication or surgery, if your injury was as bad as you make it sound.
Seeing an M.D. will significantly affect the value of your injury case. You might find a chiropractor, a physical therapist, or even an acupuncturist to be effective in alleviating you pain, but you should still see an M.D. doctor at least once or twice. The first time should take place as soon after the accident as possible for general evaluation and referral to specialists, and at the end of your treatment, to see if you need any kind of other treatment and evaluation, if you still have pain symptoms, or if you developed a new kind of symptoms. For instance, if, since the injury, you started developing headaches or double vision, you should definitely report these issues to your doctor, who then might urge you to see a neurologist.
To make sure you avoid other mistakes that would prevent you from obtaining a fair settlement for your injuries, you should consult with a personal injury lawyer to discuss your accident, your injuries and how you should go about handling your claim, whether you do it on your own, or whether you retain an attorney to represent you.