I dont think it's an NF thing. I JUST went through this exactly as you describe. Rejected by a guy. You know how I got out of the horrible self-doubt and insecurities? Self-talk. Literally. I would repeat to myself time after time: "I'm so friggin awesome!". If I had a moment of doubt, I would get those positive thoughts back in my head.
I would walk around work with my head held so high, purposely, even if I WASNT feeling all that great, but it made me exude positivity and confidence. Then I went around complimenting everyone, including the guy that rejected me. I compliment him like I'm friggin Penelope Cruz doing him the favor of a compliment! THAT's how confident you have to be. Making people feel good makes you in turn feel good. And trust me...it takes practice at first.
I didnt think it was possible, but I have shocked myself. I never knew it could be so simple.
Something a friend brought to my attention is that I'm apologetic even when I dont need to be. So now I'm more careful about apologizing. Make yourself feel worthy, even if you have to literally tell yourself that you are! Only apologize if ABSOLUTELY necessary.
Now repeat after me: I'M AWESOME. I'M AWESOME. I'M AWESOME. I'M SO FRIGGIN AWESOME!!!!
And dont just say it! BELIEVE IT! Because you are!