You should charge your Apple Watch every day. You should only stretch for longer than a day in emergency or unavoidable situations. And then it is wise to put the Apple Watch into "Low Power" mode to conserve battery.
Allowing any Li-Ion battery to drain down to just about nothing is the fastest way to destroy any Li-Ion battery.
If you have not been charging your Apple Watch everyday, then you may have damaged your battery.
If you are not at least getting 18 hours, then you may need to replace it.
Check your battery and charge your Apple Watch - Apple Support
Also see
Apple Watch - Battery - Apple
NOTE: I had a Series 4 that lasted for 6 years. It was only in the last few months that I would need to put the Series 4 into Low Power mode during the day. I charged it everyday, except in emergency and unusual situations (which did not happen very often during the 6 years).
This past Fall, I upgraded to a Series 10, and I charge it everyday. When doing sleep monitoring, I charge it while getting ready for bed, and I charge it again in the morning while getting ready for my day. It is working nicely.