Mmc mistakes fmc for his mistress

Hello everybody!

I am looking for a historical romance, in which he mistakes the fmc for another woman (he has an affair with that woman) , pulls the fmc into a room and has s x with her

Only to realize she is not his mistress

And they are then found in this position and he has to marry the fmc now

Something like that. I cant for the life of me remember what the title was.

So does anybody know what I am talking about?

Thank you