Unacceptably slow speeds

I pay for 50 down 10 up and am getting less than 10 down. This is absolutely unacceptable considering that I recently upgraded from 16 down to 50, and yet I am still expected to pay 100% of my bill. I am about ready to leave Comcast Business, as my experience has been far from pleasant. I have been told by support that my gateway would probably have to be replaced for the 3rd time now in less than 5 months. If my gateway has to be replaced for my internet to stop being throttled then Comcast will be losing a customer for sure. What can be done?