I'm going to tell you something I bet no-one has ever said to you before: It's good you're finding it hard to forgive. 

Forgiveness is hard because you're dealing with a real hurt. If you're finding it easy, then either what you're forgiving is trivial or you're living in denial. Finding it hard means you have a greater awareness of the painful consequences of another's disloyalty, betrayal, or abuse.

My advice is to find a trusted person or persons to share your story with. Talk about what happened, shed tears, express your anger, and complain about the injustice you experienced. This is lamenting. Too often we try to jump straight to forgiveness when what we require first is a witness to our hurt. 

You're never likely to feel "ready" to forgive, so you've just got to start at some point. I recommend thinking about lamenting and forgiving as a cycle. Choose a lamented part of your story—this can be a small thing, not just a big thing—and forgive the person for it. 

Then repeat.

Lament, forgive, lament, forgive. You get the idea.