Try pressing the crown and button together for at least 10 seconds to see if it’ll hard reset.
Beyond that - if the battery is no longer holding its charge, then it’s $100 for an Apple Store replacement, in the US at least. For that much money, it’s probably better to save towards a newer & faster watch and maybe get a small trade-in on yours. I know that’s not a good answer when you’re broke, but Apple watches just aren’t easy to repair.