First, to avoid confusion, a female is spelled fiancee (2 e's).
On the rest, you've made some common but deadly mistakes. Living together, engagement, planning to buy a home....this stuff is fine once you're married, or maybe in a long term, mature relationship. When people do these things because they think it's proof the relationship is advancing it blows up every single time.
You say she's been insecure all along, but this sounds like more than insecurity. Did you think getting engaged would fix this? IN other words, your ex is a nutcase, but if your relationship was where it should be, this wouldn't matter. It matters because it isn't where it should be.
I don't know what to tell you. By the time I got engaged to my husband and moved in with him a few weeks before the wedding, none of this was even possible. Nothing or nobody could have come between us.