Obie Matos

I have a 2015 suburbam with a 6L80 transmission and have been having issues with it for about 18 months, to sumarize at one point car just stoped and would not shift into any gear and i belived it was basicaly a transmission replacement job. However i managed to get the car going again and drove it home, barely made it. Once at home dropped the pan and replaced the fluid and filter. No probelms for about 1000 miles and it started to whine while shifting into 3rd to 4th gear and severely overheating also. This time i took it to a garage and i belive they replaced the transmission Pump and the car was running ok but the transmission was working like the overdrive was on sometimes always 1 gear up. This went on for about 9 months and now the winning is back and the transmission is misbehaving badly. I am going to replace fluid and like all the other times it will go away for a while and come back winning. I would say that the winning is related to oil degradation as is no longer overheating. The question is what is degrading the transmission fluid? Any comments would be helpful.