Battery on Iphone X died in 15 minutes, and now it's in a reboot loop

A few weeks ago, the battery on my Iphone X died after 15 minutes, even when I picked it up after charging the whole day or night. The battery health showed 100%. A few days later, this still happened, along with a new issue: my phone restarted itself every now and then for no reason. As of writing, my phone now is on a reboot loop and can't get past the apple logo. Why is this, and what should I do about it?