AH:
Can you explain a bit more? Are you looking for dramas where the FL is using the ML as an emotional substitute for herself while she's in love with someone else, where she's using the ML as a substitute for the guy she loves in order to achieve some kind of goal (the guy she's in love with has a role to play and she needed a replacement for some reason), where she's using the ML to make the guy she loves jealous, or where the ML is pretending to be the guy she's in love with and she finds out?
Are you looking for something similar to a particular drama?
I'd take emotional substitute to alleviate the loneliness or she's using the male lead to make the other guy jealous. Like, if its emotional substitute, the ML is aware of her feelings for the other guy, but loves her and stays with her. So he knows he's a substitute. If she's using him, then he doesn't know he's being used.