It sounds like you were trapped in your thoughts and lingering emotions. I'd wager that most of your fear and anxiety is not truly about losing her the person, or how untimely her choices were, it's much more down to losing what you subconciously built up to be the heart and soul of your livelihood.
Lot of the time in challenging relationships, the amount of compromises and times you just convince yourself to entertain something because you don't have a worthwhile alternative is huge. Most people lie to themselves to preserve the idea that they're in a completely beautiful and harmonious relationship even if there are clear and difficult flaws within the human connection between the pair. These problems only get embraced when there's good reason to, otherwise it's swept under the carpet or unsuccessfully worked upon. Tends to lead to major issues down the line and your relationship sounded like you were willing to take on that challenge with her for life. But now that you seemingly don't, embrace the truth that you were not with the perfect sort of person for you. The unsuitability and situation proved too much for the commitment to last forever. You have the license to actually admit that now wholeheartedly.
If it didn't end now, it could have ended at a different time under different circumstances. People should always be prepared for their other half to have a change of heart about them tomorrow despite what the situation is today. Things develop quickly.
I'm sure you've already done this, but if you assess how you would actually react or feel if she explained everything (as honestly as you could), you may still not feel any better or different than what you do now. The why probably doesn't matter to you as much as you think. It's the how could this happen and the what now that's likely panicked you. You probably feel like a huge part of your life has unforgivingly been plucked away from you and you're in turmoil. That exaggerated pain will pass though and it will always seem a lot deeper than it actually is, as it does for everyone in that situation. Don't be hard on yourself and do things that make you happy to help ease the anxiety.
Calmly and patiently stride forward in your life through fitness, healthier eating, sport, education, skills, new work, whatever that can advance you physically and mentally. Think less, do more, stay busy and hungry. You don't need to turn hateful, get drunk or do anything crazy to motivate you. You don't even need to be meeting new people right now. Just push on every day like a champion version of yourself. Life can be made a million times simpler or more complicated through our own self-conditioning. You take steps towards a better self and very soon you'll be revitalised about your existence and you'll enjoy pushing yourself every day. Don't even worry about what you've said and done before. Just one month of doing things better than you did before will make a big difference. When you're a new person that old shit stays irrelevant and in the past.
Health, order, balance and stability can actually make all this a positive experience in the long run. I bet you'll laugh your arse off at how much you were losing yourself over this in the near future.