Your wife cheated, you did not! You are grieving for the relationship and having a crisis of self-esteem. You are a good chooser of what you want and did well! She "was" sweet and innocent but SHE changed. You cannot control that, you can only control you and what you want and yourself/your judgement at the time. No one (especially you) knew she would change in this way, that she would meet this man. She was sweet and innocent and wholly inexperienced in life and the ways of relationships and how to have one. She had no "practice"? That's not to say she did not know any better, did not know she was breaking her own marriage vows, that her word would become worthless, that she could hurt you in this way.

Continue to work for your daughter's well being and stability and for your own health and happiness. Work to gain back your self esteem -- do you want to be wanted by someone faithless, without good character? No! You deserve better.