Unfortunately, if you have already signed a release form with the insurance company, you’ve lost the ability to pursue legal action against either the at-fault driver or their insurance carrier. However, there’s a possibility of getting further compensation if you can prove that the insurance company probably knew your injuries would require more treatment than you initially received. Ask a lawyer about “bad faith” settlement lawsuits.
Of course, the best possible scenario is to avoid this situation altogether, which you can do by not signing the release form until you have absolute medical clearance from your doctor that you are finished with treatment. Your settlement offer should include financial provisions for any follow-up care you’ll need in the future, as prescribed by a doctor.