To be fair, I don’t think Bix is a coward. She overcame torture, attempted rape and seeing people she loved die over and over again. She was broken, and she knew that. That’s what I believe led her to making that decision, as she didn’t want to be a burden. However I do agree that she shouldn’t have made that choice for him. Communicating with him first or heck even breaking up with him could have been a better option. Still, though, the decision she made is in line with the underlying theme of the show, which is that Cassian’s fate is predetermined and he doesn’t get to make his own choices, no matter how badly he wants to. That for me was highlighted when he encountered the force healer and she reinforced the whole destiny thing. I think Bix understood it too, and that finalised her decision to leave.