IPhone 11 Battery Health Rapidly Decreasing

I have had my iPhone 11 since November, My battery health dropped from 98% to 96% overnight. I had an iPhone XR for over 1 year prior to getting this phone. My iPhone XR did not drop below 100% battery health in the whole time i had it. I need my phone to lasting for atleast 6 hours however it barely lasts 4. Today i have used my phone for 1.5 hours and it is at 56%. I have done everything I have found on the internet to try and help the issue however none of which have helped. How can I get this issue fixed without purchasing a whole new battery every 4 months? I also only charge my phone at night so it gets fully drained then fully charged and this has not helped anything. My mum also purchased an iphone 11 at the exact time I did and we use the same chargers however my battery health is at 96% and hers is still at 100%.