Sorry to hear this, but it is a story we hear all too frequently. Really hard to say why this thing went side-ways, but it does happen for any number of reasons. However, there is a trend in situations where the man gets completely blind-sided, especially in circumstances were there is a live in situation.
I know a lot of dudes where something like this happened, where a man says "Mike, I don't know what happened... we never fought... no arguments... everything was fine. Then I go home and out of nowhere my girl hands me walking papers." Another twist on this, if she is living with you... she packs up her stuff and moves out leaving only a note that says "Sorry".
Okay, I'm not saying this is what happened to the OP, because there is not enough information to go on. But if you are in a relationship with a chick, and you NEVER argue, well.... this is NOT healthy. There has to be SOME tension, women are never completely comfortable if there isn't some disagreement now and then. Women NEED to emotional roller-coaster. Relationships where there is no disagreement, you always have one or the other ALWAYS giving in to keep the peace.
If the man is always giving in, well... the woman just sees him as weak. If the woman is always giving in, well.... her frustration just builds and builds... Chicks are like volcanos, they will keep this sh1t inside... there is little indication that she is building resentment until the top blows off. You have to vent this now and then by being a little selfish and not giving in all the time, or press her when you sense she is holding something in.... You have to drag this out of women. If she says "I'm fine" and you have any amount of emotional intelligence you will know she isn't fine. You have to pull this out of her.
The simple truth is that once this is on the surface, you might discover that what she wants is not something you are willing to give... at that point you have to pick up your balls and leave, but if it is something you can negotiate, and something you can live with... then go ahead and do it.
@oldmanofthesea makes a good point... you don't want a relationship for the sake of having a relationship, you have to find the RIGHT relationship.
Now advice to the OP... and something which is oft repeated. You need to move on and try to forget about her. Really, you can not dwell on this at all... if you find yourself thinking about her, change the subject in your mind and get busy with something productive. There is no magic pill, you are going to feel pain and resentment. But you have to let this sh1t go. She wasn't right for you, now just go out and work on being the best version of yourself you can be.