Can I sue an insured if unable to agree with insurance?

In July of 2017, I had a woman fall asleep at the wheel, run across the front landscaping of my home and ran into a 40ft tall tree. This tree is now dying. I have filed a claim with their insurance company (Geico) and been dealing with games for nearly a year. I finally chose to accept a very low quote they gave me which did not include the $15,000 to plant the replacement tree (huge process...need to use a crane to plant this 12,000 tree) which is only 1/2 the size of the current tree. Now that I have accepted the lower settlement, they came back to me with a $3,000 reduction in payment! I'm at my wits end dealing with this insurance company. Can I sue the person that ran through my landscaping if I cannot come to conclusion her insurance company? I'm in California.