During sex with a loved one, I sometimes get overwhelmed by fear and want to disappear. What is this and how can I help myself?

What you're describing sounds like emotional activation or more intense flooding, when the psyche perceives what's happening as a threat and partially shuts down the 'thinking' part of the brain. This doesn't mean something is wrong with you: this is how survival mechanisms work, and they can kick in even in a safe environment, especially if there's past traumatic experience. It's helpful to make a personal plan in advance: write down in first person the steps that help you specifically, gather anchors like a playlist, a blanket, or tea, and a list of safe activities if sex has to stop. Discuss boundaries with your partner in an 'if..., then I...' format and agree on what touches and hugs without intimacy help you calm down. In the moment when it overwhelms you, you can use grounding through the senses or 4-7-8 breathing, and if these states recur, consult a psychologist.