Cheated on?

I recently discovered via text message that my girlfriend of two years, whom I thought was my soul mate had in fact recently cheated on me. Upon confrontation she’s incredibly apologetic, cries, claims she doesn’t know why she did it, but that she wasn’t thinking and it was a stupid mistake, and that she is madly in love with me and want’s to spend the rest of her life with me. My world has been turned upside down… help me.