I think it's the first choice... the "trying to be shrewd, thinking she will change my mind". The first time, she kept it in and went along with it, hoping you'd change your mind in the meantime. When it happened the second time, one of two thoughts probably went through her head: either a) what he's saying must be the truth, or b) he's just saying that because he doesn't feel for me (which I think is the higher possibility by far). It may be time to just say what you told us... that you just don't want anything serious and it's not because of her. This conflict of interests, though... it sounds like this will lead to the end of the relationship.

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