How I see it now is, she wanted out, her meeting someone new was an inevitability. Why does it matter the timeframe? What timeframe would have been acceptable? 3 months, 6 months? It's just a number, therefore it doesn't matter. The time it takes them to move on is neither good or bad, it is only your perception that makes it so.
When you live with someone, there is a huge motivation to keep the peace. Everyone wants peace in their lives. If you blew up over every little thing that happened between you, you would both be miserable.
As such, you push things down and gloss over them as you live together, and the consequence of that is that resentment builds.
If resentment builds too much over time, eventually it becomes "too much" and people start contemplating an exit from the relationship. During this period, the relationship is really "on trial" but the other party is usually totally unaware of it.
Once the trial is over and the person has more or less resolved to leave, you're on the tail end of a year’s long process. It goes "things are overall good, but this stuff is annoying" -> "These things are really annoying but not bad enough that I want to leave" -> "These things are really annoying and I don't know if I can stay" -> "These things are really annoying and now I have to get out"
Unfortunately, in many cases the "annoying things" were never even articulated, or if they were, not with enough gravity. Once the "I need to leave" point is reached, whatever those things are get magnified and new ones get invented to help convince the departing partner that they are making the right choice, fort an act in self-reinforcement which sometimes requires lots of fabrication.
Over time, you have *trained each other* what to expect from the other. She knows how you will react to any given situation, what you will say, how you will act, and she has decided that's not compatible with what she wants.
