I heard this in the movie " Waterloo Bridge".
In the movie, Vivien Leigh fell in love with Robert Taylor, and then, as I recall, he had to go off to war. As you know from seeing the movie, Vivien's character turned to "entertaining" men to make money. I can't remember why she stopped loving him, but she uses the past tense, "I loved you" because she is telling him she once loved him but not any more. (As it now comes back to me: she is just saying that she no longer loves him as she realizes that if they rekindled their relationship, and Robert Taylor found out about her recent past, he would probably reject her (such were the morals at that time!). So, even though she does still love him, she tells him she doesn't so that he believes it's all over between them. But she can't be totally cruel to the man she still loves. She tells him that she did love him, and that she has never loved anyone else since. Because she is still in love with him, she cannot imagine falling in love with any other man in the future, and so she can truthfully say, "I never shall. That's the truth."
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