I had the same problem as you. But that was due to the fact that I cancelled my contract with EE on my watch. Have you checked to see if any of the apps are still open on your watch, that can Drain your battery
Have checked them all and turned off some of the health options ect. But nothing has changed on the watch. It just won't last the day. Was going to just upgrade but I fear the new galaxy will probably come out if I do lol.
you tried resetting it without a backup? if not, you can replace the battery yourself but it will get rid of any waterproofing on the watch. you can actually get a battery that will last longer than the original.