@Nortyl02,
May I ask how old you two are, approximately? I'm a semi-believer that people can benefit from having had a "wild oats sowing" time, that people who either marry or express devotion to partnership when quite young will at some point want to break out, and "test what is out there", realizing that life can be extra long if you are wanting to explore, but are stuck. Had you made each other promises?
After six years, you were thinking of marriage? Or had you talked about it before?
I'm not saying that is always true for people who got together early and then stayed together forever, but I think it is sometimes.
That may have nothing to do with you two, or, maybe it does.
Forgiveness? You aren't married, even if you were thinking you were partners all that time. It might be helpful if you two talked more with understanding coming from both of you, not just you struggling with forgiving her for what sounds like a night of fun.