JohnNeedsHelp

I have a 2015 Chevrolet Silverado and have noticed that my fuel mileage is 1.1 mpg. I hit the gas and it feels as if it is not getting the correct amount of gas to the engine. When it shifts down it shifts down harder than it should. I recently put a new motor in it a year ago and it only has around 50k miles on it. The check engine light states it is the O2 sensor. I have changed the sensor, fuel filter, and a couple of odd end things and still not fixed the issue. I was told that it was the computer and purchased a new one and when it was installed the vehicle did not accept it and would not work (it was the correct part and number I double checked). I was told it was an issue with the internal part of the motor, I can't remember what it was, and one day while it was raining my headlights began to flash while the vehicle was completely off. I have been told by three shops that it was an electrical issue and could be a short or crimp in the wire. I have had this problem for over a year and it has been to five different shops and no one can fix the problem. Any help or things that I can try to fix the issue?