I'm so sorry you're going through this :( I can see how you genuinely love him with all of your being.
Love is complex. We can't sum it all up. In this day and age, we are surrounded not only by temptation but with people who have skewed ideas about love and relationships. Single and strong-willed people out there might say that an open relationship is possible as long as you and your partner agree to it, but to a person who has completely loved their partner, like you, we know it's simply NOT possible.
I agree with you. Nothing hurts more like feeling you've been compared to and getting stuck with having intense feelings towards someone who you think has started to feel differently about you. Nothing happened, sure, but you already felt cheated on. Is it fair? I'm not sure. But is it normal? That I can say.
Take time to sort through your feelings about yourself, your partner, and your relationship. Consider the long-term impact (both staying and leaving) and any potential regrets. Picture yourself ten years from now. Are you happy? or lonely? or single? or committed?
Lastly, is he a part of the future life you imagined for yourself?
You've got this!