After 8 years you haven't married her yet but you've "been going to ask her" makes me think that perhaps she's not the right one for you anyway. Living together and never planning on marrying is one thing but to stretch a "courtship" out that long when you both did want to marry tells me that there is a reason why the deed never got done.
Up to you if you want to stay with her and work this out but I'd suggest you get some couples councelling to learn how to re-establish trust and how to keep your emotional bond strong and healthy. That way neither of you will take one another for granted and you'll both learn about personal and relationship boundaries that will keep you from falling into this kind of thing again.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion