How am I inadvertently abusing it? I have barely any apps or storage on my phone. It has a lot less of anything compared to most users. I've already tried getting rid of the Facebook app and Messenger before and it didn't help much.
You mentioned in one of your earlier replies that you're not tech savvy. We've all been there and made mistakes, which is how we learned. Most people in your situation also don't understand how to best treat Li-ion batteries. In the past, it didn't matter much because they were cheap and user replaceable. Now that all the flagship phones have sealed batteries, we're getting a lot of complaints about battery performance going bad and users not knowing what to do.
In the majority of cases, people are not following best practices for Li-ion batteries, which is to try keeping it between 40-80%. They are either assuming they can be used as a deep cycle battery like a Ni-Cad battery, or simply not use to paying attention because it didn't use to cost so much to replace (not to mention the time factor). Keeping them at 100% and draining them completely both can have a large negative impact on overall battery life. Three years on the OEM battery is actually above average for casual users, but I'm guessing by your replies that you've been going well outside the recommended ranges. You can find out more info here.
https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries
Also note that I completely ignored your comment about barely having anything on your phone. That's because it doesn't matter if the battery itself is the problem, as I suspect.
You also were asking about how to get into safe mode. B. Diddy provided a link to instructions in post
#5 of this thread. Yes, you shouldn't have to do that to simply use a phone but it's all part of the diagnostic steps to find the culprit. If you run it in safe mode and still have battery problems, then all this focus on Facebook is for nothing. At that point it's either something glitched with the core components that would likely require a factory reset, or the battery.