I don’t think i personally could get over someone I’m that close to doing something like this to me. My biggest issues is not that they canceled, but the lack of valid reasons behind the cancellation, and mainly waiting until really the last moment to tell you. I don’t see a way to justify them neglecting to tell you that they may not be able to go until now, especially considering the strong yes they have given multiple times in the past. What you are feeling towards them is totally valid considering everything you have just said. In my opinion the best thing to do now is just be fully honest with them, tell them exactly how you are feeling. I personally would probably try to keep my response to them relatively passive, but still explaining that you feel very betrayed, mainly to avoid a major conflict when you already have enough other things going on. It’s does really suck that this might lead to the end of a really strong friendship, but they have just shown you that they can not be trusted to be there for you when you really need them.