Extreme Pro Internet @ less than 100 Mbps

I need help. I have the Extreme pro Internet which is up to 500 Mbps, and Ive been running the xfinity speed tests, and online, and Im only getting 100 Mbps, just over the last couple of days its gone down to abiut 35 to 40 mbps. It drops connection, cant seem to stream anything, trying to get school work, and working from home is almost becoming impossible because of the slow speeds. Ive chat with xfinity and they reset my gateway saying they detect problems, Ive tried to talk with an agent, they reset it then hung up on me, and now all I get is xfinity saying they are prioritizing to just those with loss of service, however, this is a loss of service because its not working properly, and definitely not at the speed Im paying into it for. Ive never had problems with my services before, but something is wrong.Could someone please help?