This was never going to be a 'good' romance story. It was never going to end well. Being mad a story isn't something entirely different that it was never meant to be is a reader's prerogative. As I said, it's a second act story, in my opinion.
Diana COULD have a long-lasting love. This wasn't it, that's all. Spider-man COULD have married Gwen, but he didn't. Batman COULD have decided not to let Silver St. Cloud go, but he didn't.
I submit that tragedy and loss is part of a hero's journey and if they don't have it, success is meaningless and shallow.