I had the same challenge with my son who loves his routines which included me sitting with him until he fell asleep. When I got tired of falling asleep too at 8pm I decided to ask him what he needed to be able to fall asleep on his own. He said listening to the radio would help so now he listens to music (not the radio as I hate commercials but soothing songs), I go in an hour later and turn it off once he is asleep.
Hope this was helpful.
ps - Any other changes in his "routine" and he tends to revert back to wanting me to stay with him but it doesn't last too long.