Your doctors are the only ones who can determine if you have reached MMI. They have the expertise to evaluate a patient and run tests to determine where the victim is in his or her recovery.
If you disagree with your doctor about where you are in your recovery, you may want to seek a second opinion from another doctor. Maybe you think have more room for improvement or that your injuries are worse than the first doctor says.
It is also important to note that the insurance company may request that you have an additional medical exam. You will be sent to a doctor who most likely will not agree with your treating physician. Many of these doctors are paid up to $500,000 a year to write reports alleging that your injuries are not as bad as your treating physician says.