Left screen is darker

It’s already 3 years old, as you say, so its probably accumulated dust in the FPC cable contact pins.

Try opening it up and cleaning it thoroughly, see if it fixes the problem.

The cables should look somewhat like this

https://sc01.alicdn.com/kf/HTB1YB9eHXOWB...

If it does not, your board is most likely at fault, meaning you need to get it replaced or buy a new TV altogether (sometimes the later saves you tons of time, effort and risk for little monetary difference).