Feel of Dread and Panic

I (F27) have been in a relationship for 6 years now, and sex started to fizzle down about 3 years ago. My relationship is healthy, never harmed me mentally or physically. I feel overwhelming panic when sexually intimate. Like my body does not know the difference between receiving oral and being held at gunpoint. I love and trust my partner so I do not know why this is happening. He is very understanding so we don’t do it often, but the most recent time was the worst. I felt like I was having a panic attack, nauseous, numb. I don’t know what to do, my doctor suggested seeing a therapist but my insurance doesn’t cover it enough so its still stupid expensive. I’m just tired of this feeling and feel helpless and dumb.