I have a partner. he is the most endearing person in the world. no one has ever been able to understand, accept, and love all of me before. for me, it was love at first sight. the feeling had barely changed aside from overcoming the honey moon phase (which was pretty awesome.) he is literally the most perfect girlfriend a gal like me can have.

but a while ago, he had broken my heart.
it was in the very beginning of our relationship. he had informed me the reason he was even being apprehensive is because he was in love with someone else who didn't love him back. eventually our relationship started, but his feelings never went away.
it was about a month in when I had confronted him about the messages they were writing to each other. there was a lot of shadiness that happened. he said he didn't physically cheat, and of course I believe him. but he was still "fond" of her.
that was the last time he had ever talked to her. and I know his feelings dissipated quickly after that. since then, he has been the most loyal person in the world. that was almost 3 months ago.

so, why can I not forgive him? I find myself staring into space sometimes, letting that moment play in my head. I'll get into a very deep depression over it. sometimes I will occasionally snap at him at the most random of times because of it.
he is not the same person he was in the beginning. he is a changed man. but I am still the same girl, broken heartedly resentful.
how do I get over this and forgive him? I feel like if I let this emotion stay, I will always be poisoning our relationship. what we have is perfect now, and I don't want to ruin it. how can I forgive him?