Tips for vulva-on-vulva sex

I have no idea what I'm doing. Everytime I go down on my girlfriend I feel like I'm not doing anything right. One time I was and she starts laughing. She said it was because it felt good but it ruined my confidence. That was three years ago and I haven't been able to comfortably do it since. I always end up hurting the underside of my tongue, which I've read means I'm doing it wrong. She doesn't like to be penetrated so fingering and vibrators are out. We've tried tribbing, which has felt good, but I have a hard time moving my hips in sync with her. I feel like a failure in bed and it makes it so hard fo go into sex with confidence. She does everything so perfectly. I feel like an inexperienced teenager again 😣Any tips would be great...