Well, it makes sense. You didn't want this break-up and are desperate to stay in her life in some way.
She is able to compartmentalize because she doesn't have the same sense of desperation or fear of loss hanging over her. She's got both you and her ex vying for her time and attention, so she's relaxed. She can pick whomever she wants here so she doesn't need to put things out of her mind, so to speak. That would suggest she is in pain, but she's clearly not.
You can't. You are seeing why that doesn't really work.
This why we have been telling you that staying close to her is not a good idea right now. It stings when these scenarios don't play out the way you hoped. Please reconsider being her surrogate boyfriend.