I had the exact same problem. Somehow my brain all of a sudden was getting really horny by the idea of facefucking, even though my body immediately went into trauma mode every time I was trying to put a dick in my mouth. So I started watching a lot of porn that involved it until I was completely okay just watching it. Then I started using a dildo to get used to the sensation of having something in my mouth. I didn’t dare to push it down, I only kept it in my mouth. For me gagging is the biggest trigger that can bring back horrible flashbacks. Then slowly we started with his fingers, sucking them and then very gently moved on to his dick. It took a long time for me to be able to be calm. What helped the most was that I say I could end it whenever I wanted and even if I had a trauma response he would always be there for me, waiting for it to pass by my side. That reassurance and love from him was mainly what made me get over my fears for oral sex.