How to get refund for being overcharged for a year?

There's no way for them or for the system to raise the internet bill if you're already paying for the internet's regular price. You must have lost your bundle discount that's why your bill went up. 

I am paying the internet regular price. there is no discount. I added TV. now when I take it off the quoted internet price is higher. I argued on the phone for 15 minutes and compared bills, then was put on hold for almost 9 hours. 

What internet speed did you have before? Price? Which gateway? Equipment fee?

What Internet speed do you have now? Price? Which gateway? Equipment fee?

How long did you have the TV package?

When TV service was installed, you received a new bundle pricing for internet/tv, which normally includes an extra $10 off internet package compared to a stand alone internet package. Thus canceling the TV portion would raise the internet bill $10. Additionally, depending on your internet package, you would now have an equipment fee of $10, which you might not of had previously.

Lastly by terms of service agreement, lowering or canceling a part of bundle would result in remaining service(s) going to full retail price.

In case you did not read the TOS you agreed to when establish TV service....

https://www.att.com/legal/terms.uverseAttTermsOfService.html

Changes to Fees and Charges.

If you signed up for Services for a specified term, you agree that if you cancel your plan before the end of the term, you will pay any applicable early termination fee. At the conclusion of your term, AT&T will automatically begin charging the applicable month-to-month fee. If you purchased the Services as part of a bundled offering with one or more other products and are receiving a discount based upon that bundled offering, your discount may cease and you may be billed the standard monthly rate for the Services if you change or disconnect one or more of the services in the applicable bundle. AT&T may, upon notice required by applicable laws, at any time change the amount of or basis for determining any fee or charge or institute new fees or charges.

Retail prices of internet speed tiers.....

Pro 3M $47, Elite 6M $52, Max 12M $57, Max Plus 18M $62, Max Turbo 24M $72, Power 45 $82

Equipment fee new installs since June 2016 $10 in addition to above prices....

Previous (grandfathered) equipment fee $7.

All new installs (adding TV service is a new install) or gateways replaced on repair calls after January 2015 has the equipment fee. 

I added them at different times. I receieved no additional discount on my intnernet when I added TV. 

They have me use a new gateway for the TV service, I retained my previous one that I purchased outright and did not have from a monthly payment, so when going back to internet only I will be using the gateway that I own. I had a non-TV related discount on internet that continued after I signed up for TV Service. I had to renegotiate that internet discount during the TV service but at a different time than signing up for TV service. Now I will lose that separate discount when I cancel TV even though thta discount is the same one that I have been getting for multiple years now? 

either way your system is messed up and this is a case not following what must be some kind of standard procedure and packagaing. this is ridiculous. before TV I had 0 equipment fee. Since I will be using my own equipment for internet after cancelling TV there will be 0 equipment fee. There is currently a non-bundle related discount on my internet. They are trying to take away that discount when I cancel TV.