battery percentage dropping insanely fast???

i've had my iphone 11 for just over a year and have had absolutely no problems with the battery until a couple of months ago (coincidentally around the time i started playing genshin impact for hours a day on it 😰) when i noticed my battery health had dropped to 81%. it was still doing fine until this week i noticed the battery health is now on 79% and since checking that my battery has been dropping from 100% to 10% in what seems like minutes depending on how often i'm using my phone. also at the time of posting this my phone has been on 1% for around 4 hours now and i've been using it constantly almost the entire time ????? does anyone know what could be causing this/whether there's a simple fix for it? and i'm aware i'm going to have to get the battery replaced eventually but as a 21 year old currently working minimum wage i'm not sure i can just drop £70 on a new battery right now 🤦‍♀️