Obsessing over intrusive thought

I had an intrusive thought about a family member while I was being intimate with my boyfriend now every time I think about my boyfriend I see that family members face and go into such a panic with feelings of disgust towards myself I don’t know what to do. My relationship is strong, with a man who cares for and loves me, but I feel like I’ve ruined everything. I just cry and obsess over it, I haven’t given myself time to recover I know this, but I’m so scared of losing this person because I feel like I don’t have any feelings towards him because of this, when a couple of weeks ago everything was perfect. I’ve been prescribed sertraline by my GP. I’m reading CBT based self help books but I need something that relates, please can anyone help me.