Help with negotating with an auto adjuster

I was a passenger in the car. The car hit a guardrail head on going about 40mph. my head busted the winshield and my knee hit the dash. I was taken to the hospital where I was treated. I have been under doctors care for a year now along with physical therapy ,pain management, medication for treatment. All of my medical bills and time off work come to 30,000. I think that the settlement offer that they are giving me is way to low I of course have counter offered back twice. He has made a third counter off which is still to low for what I think I should recieve for future bills and pain and suffering. I have sent him a demand letter after he made his first counter offer. What is the best way for me to go about this to respond back with the negotiation process to get what I deserve. I want to settle this on my own without getting a lawyer. Any advice would be appreciated.