I agree with just about everything mattpd just said, except that there is value in asking "why" -- not years of suffering over it, and not trying to understand his justification for it, but rather, why do you think he did it -- why do you think he broke his promise. Have you asked him why he did that? Did you listen generously to his answer, and give him some time to think about it if he needs time, because it might be the first time he's asking himself why?
And most importantly, taking 100% responsibility by asking yourself about "why."
Not like it's your fault, but rather, taking a bold look at who you have been being in the relationship and seeing if you can get insight as to why the relationship is occurring
for you the way it is.
Which is very different from wringing your hands and wondering why oh why did he do this to me?
It's more like: why and how am I doing this to myself?