Who is at fault?

Yesterday I got into a fender bender. minor scratches and dents on both cars. I was stopped at a left merge sign on the highway, as I didn't see it due to a truck in front of me. I turned on my blinker for at least a minute to indicate my merge into pretty slow traffic, I saw a big gap between two cars and as I was attempting to merge slowly, the car directly behind me in the same lane tried to cut in front of me at a higher speed than I was going, as I was already in motion to move into the lane. They clipped my driver's side mirror/fender with their rear right tire as they were straightening out into the lane. Who is at fault here?