I definitely fall into the category of agreeing that the Matty thing was questionable, icky and too soon, but thinking that Taylor is going through the biggest break up of her life and having to perform to hundreds of thousands of people every week at the same time.
Her choices of secret songs definitely point to the ones about Joe just being too painful to sing about. The ones on the main set list are different. She's not as vulnerable there. She moved on quickly and the public declarations were weird but that definitely looked like it was Matty's idea. From the way he has behaved I really wouldn't be surprised if he genuinely thinks he's in love with her. Even the interviews regarding her in the past were a bit mentionitisy (yes that's a word).
I married my only LTR, but I've also made some bad choices when it comes to men in the past. If I had come out of a LTR I dread to think what cringey af thing I would have done (especially if I thought my ex had moved on with a much younger woman). When folklore first came out my favourite song was MTR because I really identified with the line "I didn't have it in myself to go with grace" because sometimes I just don't. I'll tolerate so much until I just think "nah" and say exactly what is on my mind or do something petty. It often works to my advantage in fairness, but people find it shocking sometimes.
My point being I literally can't imagine how I would cope with the attention if I were Taylor. I don't think You're losing me is particularly damning for Joe. I see the "he wouldn't marry me" admission as something that would be very very hard to admit actually and not something you'd normally want everyone and their dog knowing. Taylor has pretty much spent 5 years telling us how much she loves this man, he is the one for her, he is more than enough and how she didn't want to lose him. But now she has. She wasn't enough for him, or maybe she was too much for him. I see releasing that song as kinda shutting down the speculation a bit. She was worried he'd tire of her and look, he did. It was gradual and they both saw it coming.
We also don't know the timing of You're Losing Me. Yes it sounds very recent. But imagine they had broken up in the middle of writing for midnights and she wrote that. There's NO WAY she could have put it on the album then if they'd gotten back together in the interim.
I personally don't believe she cheated but we don't know either way. Yes we know she hung out with Matty for one night in Jan but that could have just been a seed sowing/flirting/spark/friends/drunk occasion. Or maybe she was trying to make Joe jealous. Or maybe they were on a break back then. Or maybe they've been having a dirty side thing since he worked on midnights. Or maybe she agreed to appear at his concert because their collab didn't make it onto midnights and she felt bad. We don't know.