Of course it can be disorienting and I'm not questioning your feelings. But the fact is that life circumstances change and people make decisions based on that and they have every right to make such decisions.

What is relevant for you is how can you cope with decisions that others make about their lives where you have no say. Based on your post, it's very painful for you if these things happen. But these things are part of normal life and an important goal for you could be to learn to tolerate and accept these things, so that they wouldn't hurt so much.

Her promise probably was quite honest in that moment and she most probably did not make that promise with the goal of braking it. Sure, it could be said that it was a mistake on her part to tell you something like this that felt like promise to you. But anyway, things changed and life changed and now things are different. This is something you have to accept, in one way or another.

As I said, if you feel that your trust in her is gone, that's totally valid too.