On the desk in Susan Lancaster's room in the Tenpenny suites and can be shown to Millicent Wellington as part of the quest Tenpenny Tower. Note that after successfully completing the quest, the love letter is inaccessible.
My dearest Susan,
I can't believe I'm writing this... but I can't help it. I can't hold it in any longer. I can't stop thinking about you! Even when I'm with Millicent, all I see is your face! God, can a man be in love with two women at the same time?
How can something so wrong feel so right? There must be some way to make this work! To make this right...
I can't wait to see you. But I don't want it to be like before. It's different now. Because I've finally told you... I love you. This is real. This is so real I can't take it any more. I need you right now!
Yours truly and forever,