Sorry to hear that Fin.
I went through a relationship like that (not as close to marriage, but severe mood swing like that, no explanations, sometimes the 'I need time on my own' BS, severe broken heart, borderline-paranoia, etc.)
When I look back at it, I'm grateful that after the ensuing pain and recovery things didn't work out. It was a learning experience and an eye opener.
My advice, stay far... far away from women like that.
My next serious relationship was a couple of years after that one, and I met a much 'simpler' person (in terms of mood swings/personal complexity), whom I made clear from the get go that communication was priority number one. If she couldn't tell me how she felt, and she kept piling inside, then eventually it would all come out one day and we'll be screwed by then. We built a pretty solid relationship around that, and after dating for a little over a year, we moved together. We married shortly afterwards.
I have 5 years being married (anniversary this coming Saturday), and to be honest I'm probably the happiest married man alive. Wouldn't have it any other way really. But I also understand that I found the closest thing to what I wanted/needed, and that we're 100% honest to each other. We don't hold anything back. And that's what works great for us.
Ultimately, if she feels like she can dump you but she can't look at you straight in the eyes and tell you what she really thinks or how she really feels, then that's a mammoth warning light. Stay away. I understand the pain that goes through you, and I would say: try to keep distracted. Go play some basketball, keep your mind busy at work, and let time heal you. You will come out of this with a lot better idea of what you want from a relationship, and specifically what you absolutely do not want. And I'm sure someday you'll cross roads with your match. Really wish you the best bro, because I know how painful this can be. But there's always a light at the end of the tunnel.