Being cheated on: my worst fear?

I seem to have an almost paranoid fear of being cheated on. My partner ures me that he couldn't be more satisfied, on all areas of the relationship. Yet, i panic when he looks at other pretty girls or women, and i feel compared to all the perfect faces and bodies out there and on TV. I have no insecurities about my own appearance and intelligence... yet.... my fear spoils many a moment. On parties, I cannot stand seeing some of his exes, when he is at work, i wonder whether some blonde bombshell isn't swiping him off his feet. I don't know what to think!