Why do I not get the internet I pay for

Having issues with Cox, I never get the promised speed.  My location only has Cox as a provider in a large city. I'm trapped into paying higher rates to get the speed I should already be getting.  I pay for 150 download.  I only ever get 80 at best. I've been through all of the Cox system, but the final answer is, Cox only guarantees up to half of the promised amount.  They know what they are doing.  I could pay for better internet, and they would have to increase it from my current 80 dl.  Or they could just honor the 150 dl I pay for.  It's a trap, but I'm stuck and unhappy.