Sometimes it can be too easy to try to forgive and forget. I had a relationship where my girlfriend cheated on me even though we had an open relationship. All she had to do was talk to me about it, but she hid it instead. I saw the warning signs and clues that she was cheating, but I kept accepting her flimsy excuses because I loved her and wanted it to work out.
We finally broke up and I realized how badly I had been cheated on afterwards. There were so many lies in the relationship that there was never any hope for regaining any trust. Sometimes you just have to let go of someone who is fundamentally dishonest in a relationship.
Personally, I think the poly lifestyle is more about the philosophy of relationship than the number of people you are seeing at once. I think poly is more about openness and honest communication with full knowledge and consent of all parties involved. It is about compersion and dealing with jealousy as a sympton instead of as a problem. Where love is not about possession but allowing people to "love as thou wilt." So to me, a person can be single or in a monogamous relationship or be monogamous themself, but I will see them as part of the poly lifestyle if they subscribe to the above ideas.