Terrible experience. They've determined I'm at fau...

Terrible experience. They've determined I'm at fault for a car accident that happened because some moron tried to overtake me... in a SINGLE lane! I was turning left and they tried to overtake me on my left and somehow I'm at fault?! How does the other person get away with ignoring a turn signal, and overtaking me in a single lane ON THE SAME SIDE I WAS TURNING TO?! I understand the fault is with whoever turns left, but in this circumstance, the fault should be 50/50 if not 100% the other guy.