Betrayed

One minute my fiancee is helping me look at cakes and calling photographers and the next minute he is telling me he thinks that we are arguing too much and he thinks that things aren't going to work out. How does someone switch gears like that? Well apparently he had been feeling this way for the past two months or so. Nice time to let me know, the weekend I'm about to send out the save the dates. I guess I'm lucky I hadn't sent them out yet. And the best part is that after talking things out all night and talking again in the morning he still has another surprise for me. He tells me that he is going to his friends birthday party, alone. Yeah. I have been doing without so many things so that we could put money away to pay for the wedding ourselves and he just up and decides to go and go without me. Then he tells me that he just forgot to tell me that he was going, the morning of the dinner. Oh, and this isn't just outback, this is Ohana's at Disney. That's $70 dollars a head.
To top it all off he knows that my ex did a similar thing to me right before we got together. He came home and changed telling me he had a meeting for a job and ended up having dinner at another expensive Disney restaurant with his ex girlfriend.
Am I being over sensitive? Should I not be offended that he planned this dinner behind my back to purposely leave me out? Or am I justified in being pissed off?