[Aga-chan]

Well, her feelings changed. Together When... is about that. They thought they found an "unchanging thing" (true love), but saw it changing and kind of drifted apart. But since then her outlook on the matter changed. I think you should read the lyrics of the following songs:
Together When..., LOVE'n'HATE, You were..., WHY...
that should give you a better understanding.

I have a feeling, though, that perhaps their respective companies had their play in all that as well, especially his. Johnny's have to keep their relationships kind of secret and can't marry when they want to. I always thought that the elderly man giving away Tomoya's puppet to the boy in the Together When... PV kind of hinted at that, but I might be overinterpreting. I'm not saying they broke up because of his company, but that their interference might have led to some frustrations?
Until she doesn't tell us herself, and she likely won't, we can only go from what we have, though.

And everyone saying that it was because she didn't want to get married is making stuff up. Nothing ever hinted at that, on the contrary. You only have to look at CAROLS and You were... to know that. Or the pair tattoo they had.