I'm not okay </3

My boyfriend and I have been together for over a year now. Over the past few months we've had our ups and downs. I have been feeling so out of it lately. He has recently expressed an interest in taking a 2nd partner, and I am torn. Am I not enough for him? Or is it that I do not deserve someone who can love me and me alone...? He says I'm the kind of woman no man should ever fumble... so why is he okay doing something that could put him at risk of losing me? I guess it may also be my fault for promising i'd never leave... my heart hurts so much.