How do I deal with this

Okay so my boyfriends ex works with him. She keeps telling stories about giving him blow jobs to his co workers. I can't get that thought out of my head and it's really hurting me. Any advice. Btw I'm not good at blow jobs and I know it, and I know she did it a lot to him so maybe that's a factor why it bothers me so much cause I feel crappy for not being as good as her.