Why is my battery dropping so quickly? [Huawei Honor 10 Lite, HRY-LX1]

I've had this phone for nearly 3 years now so i expect the battery to be a bit worn out, but it was working without any noticeable problems until a few months ago, when it started dropping very suddenly. For example, sometimes i wake up the phone after leaving it alone for a few hours, noticing the battery is at, say, 30%, and after just about 3 minutes of use, mainy gmail and browser, it drops by about 10%. Or, other times, it drops by 5-10% overnight, while it's on airplane mode and turned off. I’ve checked for malware and uninstalled any apps that might be taking up battery by running in the background. Does anyone know why this might be happening and how i could be able to fix it? If you need clarification regarding my problems please let me know.