You are obviously more heavy footed than you realize and far more of a masher than the rest of us. But by using mostly the middle ring and smaller rear sprockets you are generating more tension in the chain than you would if you use the larger ring and bigger rear sprockets. This is purely a case of leverage. The smaller the front ring the more tension you'll get in the chain for a given pedaling pressure. If you switch to the big ring and use the bigger rear sprockets in the middle of the cassette you will generate less tension in the chain, less breakage and use sprockets front and rear that have more teeth and spread the load out more and wear less quickly.

But at this point I'd start by replacing the chain and cassete. It's seen enough injury and woe. If you can find one go for a 12 to whatever cassete since by using the big front ring more you won't need the 11 by the sounds of it. This will shift all the sizes down one position and allow you to ride with the middle or smaller rings on the cassete. That will reduce the cross chain angle a little which will help lengthen the chain life again.