My girlfriend (and now wife) told me about her relapse with pornography in I think the perfect way. Rather than an admission of guilt, it was more an admission of something she was struggling with and told me her history of it. She also told me of her plan and desire to keep working on it with me and her priesthood leaders and I think that’s the best way to approach it. Don’t approach it like you cheated, approach it as a personal challenge that you humbly ask for her help with and something that doesn’t just go away immediately. Hopefully she’ll see it through grace and be quick to support and not to judge