Even if you love someone with all your heart you can still make mistakes. Maybe there was something missing in her life, maybe she was trying to find herself, maybe she had to be sure she loved him.
Mistakes can mean nothing; sometimes mistakes can be about wanting more attention, feeling inadeqaute, and sometimes has nothing to do with their significant other.
Mistakes can change your life for the better if it makes you realise something you never knew before, a big life changing event like, wholly loving someone like you've never done before or thought you were incapable of doing.
I rest my case.