My heart goes out to you. Im interested in how her communication has been with you since telling you. Has she been readily willing to discuss everything about it? Like why, who, how? Ive been lucky enough to only have this happen (that i know of) in relationships that werent too serious. But I do know that its nearly impossible to regain trust without the offending party being completely open, honest, and nurturing. I dont agree with people who say you could never trust them again. But if they aren't willing to do all the hard work (a long time spent trying to make you comfortable in the relationship again, counseling, a lot of reassurance, a lot of uncomfortable conversations, etc) to help you regain trust then you're probably gonna have to leave eventually. I know it hurts like hell, and it may seem bleak. But please remember that people go through this all the time and eventually make it to the other side happy and healthy. You'll get there too no matter how you end up handling it. I dont know you, but no one deserves that and im rooting for you.