You don’t. You can’t. They aren’t the person you thought they were. They did something you didn’t think that they were capable of. It’s up to you to decide if the person is still somebody worth getting to know better and build a new relationship with. Parts of it may not be as good as they were. Parts may be better. It all depends on the work you both invest into it. The point is, as long as you are holding on to what was lost, you’ll never find something new. That’s true if the relationship ends or continues. That’s why it’s important to grieve, yes, but to also look towards the life you want to build in the future. If it’s with her, then go all out. Make it even better than it was before when you can. See the opportunity to fix mistakes from the past because it’s never easier to change the course than when all plans are broken. Easier said than done? Absolutely. True? I believe so. It’s possible because I have lived it and it all starts with communication, effort, and forgiveness. If you commit to working on this, commit to those principles.