My iPhone battery is draining very fast

On your iPhone, touch Settings > Battery > Battery, Health & Charging. What is the "Maximum Capacity" listed? More information about what this number means is discussed in iPhone battery and performance - Apple Support. Great advice about battery charging also exists here: When to charge your iPhone or iPad - Apple Community. If your personal daily charging schedule is regular and reliable (i.e. you charge every night), then you should also ensure that the "Optimized Battery Charging" option is enabled.

Then, visit Settings > Battery, scroll down, and look at "battery usage by app" to determine which app(s) are your biggest users. Touching "Show activity" also provides additional app batter usage information. From that information you may want to disable background app refresh for offending apps or even uninstall any that are major battery hogs.

Hmmm, what is your Settings - Battery - Battery Health & Charging maximum capacity of your iPhone? One piece of advice I could give is let your iPhone’s battery fully die, then put it on a slow charger. Hopefully that works. Also, if you have any large apps running in the background, it could be severely draining the battery.