The fact that he'd earned her trust before Eadu means that it must have hurt her all the worse and Cassian screws his mouth up, thinking Jyn won't tolerate another apology, another 'if only,' but thinking she must recognize it on his face. There's so much of his past that he'll never forgive in himself but they also have a future now. They've already begun it, long before getting married, and if he lets it be a stone around their necks forever, they won't get far.

The future.

"I promise," he says. "To always believe you." It feels a little like he's concocted a new wedding vow right on the spot but why not. If Jyn is going to prove him wrong then he can do better too. He'll never feel like a good man and he's had a hard time even believing that someday he'll be good enough but if Jyn says it and he promises to believe her...

"And," he adds, thinking of the earlier topic. "I think you'll make a great teacher."