The "in one conversation, I tore down the whole sky" makes me think she issued an ultimatum - marriage or we're over. She thought he would propose, but he considered it over and it devastated her, so she spiraled and went after Matty.
Yeah, I'm still very much of the opinion that if Joe had wanted marriage they would still be together.
If Matty was telling her that she was the love of his life every time he saw her, you can understand why she would be tempted thinking the grass was greener or that they were star crossed lovers, seperated cruelly by fate.
The lack of commitment by Joe must have been hard to take. All the usual excuses for not settling down didn't apply. They were a sensible age, plenty of money etc etc. He either didn't love her enough or couldn't cope with who she is. Even if she had given up music entirely for him (and why should she have?) she could never not be Taylor Swift.