What cause the fast discharging if battey

There can be a lot of reason. Firstly, main problem for the battery is heat. If you play games too much, most probably battery health will decrease faster. If you use it normal, there is limit for 500 cycle. After 500 cycle your battery will be poor condition because this is how Lithium batteries works. Also look at your charge. Don’t put your phone charge overnight. Also don’t let it to go up to 100%. Healthy range is between 30 to 50%. If you just use it until battery dies, it is not a good idea for lithium batteries. Because cells will die faster.