Wilks, if you were able to get a police officer to investigate an accident on private property (as the parking lot was), that is very unusual. Most police agencies won't touch a private property investigation.
In any case, as you have found out, the insurance company does their own investigation and determines who is more likely at fault. Some insurance companies are fair if their party was at fault, some completely refuse to accept responsibility on private property. Usually, the two parties insurance companies will work out some mutually agreement resolution in a case like this. However, since you do not have an insurance company to 'fight for you', you will have to do so on your own.
If you disagree with their determination, you have little recourse other than to file a small claim action against the other driver. His insurance will then retain legal counsel for him to defend against your claim.
Best of luck... it is unusual for a Pro Se litigant to succeed against a competent attorney.