After beating my brain around all possible reasons why she would cling to the 'FL is the only real love' idea, I came to a wobbly conclusion. So, we all agree that Dylan is being an idiot for thinking the way she thinks, however, as a woman who knows she's not 'the one' I feel like I can understand her a little. If I were in her shoes, getting married to this handsome dude would be the perfect chance for me to potentially fall in love with him, so rather than 'nah I'm not the right one' she might feel 'nah I'm not as good as the FL'. She mentioned before how her sister is pretty and kind etc., but she doesn't see herself as pretty and loveable, hence not worthy of MLs love? I guess? So, she might just be scared of allowing herself to think anything is possible, get disapointed after falling in love and ending up with a broken heart, IF the ML really does fall in love with the FL. It's a chance, no matter how small, and I'd say she is quite reasonable by assessing this situation, simply because no one really wants to suffer. As I said, this is a stretch, and just my opinion on her way thinking.
Take it with a grain of salt hahah. Cheers. 👍