I need advice..

what would you do if your boyfriend/significant other told you that they had developed feelings for someone else for a while, but never did anything with them, kiss, fuck, etc and shut it down because they loved you? my boyfriend told me this last night, and I'm still hurt about it. it makes me feel like I did something seriously wrong. like I wasn't a good enough girlfriend. I love him to death and I don't wanna leave him because of it, I'm carrying his child, and want to be a family. I just need to know what you guys would've felt like, did if you had been in my place.