When I have serious problems with my Win 10 I will perform a repair install which doesn't affect personal stuff or installed programs.
For this you will need Win 10 install media which you can create after reading the instructions at
https://www.microsoft.com/en-gb/software-download/windows10However, when I tried a repair install on my Win 10 1903 it took about 7¼hrs.
Did a sfc /scannow report anything untoward ?
You could open a Command Prompt (Admin) and enter -
dism /online /cleanup-image /restorehealth
do another sfc /scannow and then try the update again if that doesn't report anything untoward.
The odd time an update has failed on mine, it has installed it again straight after which has usually been successful.
Event Viewer may give you an error code for its failure which would be something to go on.
I assume you have performed at least a chkdks /f which will report if you have any KBs in bad sectors ?
To view a chkdsk log, open Event Viewer and when it has read the data, expand Windows Logs - click on Application/Action/Find and type chkdsk into the Find box.
Cancel the Find box and read the log in the scrollable window below.