Iphone 6 battery dies very fast

Over the last few months, my iPhone 6's battery has gotten progressively worse and worse. it has nothing to do with apps being open, or the screen brightness, or needing to download new software updates, or anything like that. It got to the point where if it was not on a charger 24/7, it'll die within minutes, or less, of it being disconnected. i bought a 10 foot charger for this reason.

More recently its gotten even worse than that. it'll still die even if I have it plugged in and was using it. I'm wondering if there's anything I can do besides getting a replacement phone or paying for someone to fix it. I'm pretty savvy when it comes to tech and electronics, I've done everything I can think of short of doing a complete factory reset and taking the thing apart myself.

I've never before had a battery fail on me like this in a cell phone. I've had it a little over 2 years. the battery percentage indicator on the top right will say it has like 90% remaining or something to that degree but its a lie. it'll die even with it saying 100%

thanks