I Changed The Coolant In My Car And Now It's Overheating, What Have I Done Wrong?

First up, check the coolant level when the engine is cold and the vehicle parked on level ground. The coolant level should normally sit in the middle of two markers, Min and Max. If necessary, top up as recommended in your owner's handbook and check underneath the vehicle for leaks.

If you find a leak, the two most likely places to check are the bottom radiator hose and the drain tap on the engine block. Ensure the radiator hose is properly seated on the radiator and that the jubilee clip or fastener is tight. Check, too, that the drain tap is properly closed, or if a screw in type, that it is seated correctly on the threads and tightened up sufficiently.

If there are no leaks present with the engine switched off, switch it on and let the engine run until it reaches normal working temperature. If you still cannot see any trace of a leak, then the conclusion is that there is air trapped in the cooling system.

To expel the air, carefully and slowly remove the expansion tank cap to avoid being scalded. Better still, wait until the engine is cold before doing this.

So, with the expansion tank cap removed and the engine running, locate the top radiator hose and gently squeeze it. Be careful to avoid being caught in moving engine parts. Continue squeezing and releasing until you can no longer hear or see bubbles escaping in the expansion tank. Have an assistant keep an eye on the vehicle's temperature gauge while you do this.

When there are no more bubbles popping, switch off, and then top up with coolant if necessary when the engine is cold. You may need to repeat this procedure a couple of times.

i think you have been cheated in the quality of the coolent that u have used in your car and it also posiible that u have not use the coolent according to the requirement of your car.
the viscosity of the coolent will be low and it has caused the problem
and also earlier the fuel the fuel that u are using is of low grade so it may cause the knotting and the choking and the formation of the carbon particle which increases the heat generated from the cylinder and that's why that excessive heat generation caused  is liable to the thinner coolent and it rapidly heats up!!!!