Reason for ending previous relationship does not compute

I asked a girl (the same one in my other thread) why she left her boyfriend of 4 years (keeping in mind they were engaged for 3 of the 4 years (the boyfriend proposed after the first year)). She said, "I feel like I couldn't be myself."

It doesn't really make sense since they were together for 4 years, then she leaves and her excuse is this. Usually a relationship that lasts that long comes with being able to be yourself, and not being able to do that would cause the relationship to break up before then, right?

So, what does this mean?

Computes to me.
It can take a long while for someone to realize something like this, or for them to get to a point where it means more to them than holding onto a relationship.

Take it at face value.