Hero, I know you have understandable issues related to your OTP, whom you loved angstily for 13 years.
If you are making love to a woman who asked you to, and she tells you "I love you," I believe it means she loves you. To believe that she is in love with another man and said what she said because she is imagining/pretending you are him involves a degree of dedicated and imaginative self-torture no normal person should be capable of. Snap out of it! Or not, as your angst is delicious.
I am reading Putney's Petals In the Storm with Napoleonic era spies and angst and soooooo gooooood!