Jealousy can be a vicious thing. Acknowledging that it is jealousy is a big first step. Perhaps some personal counselling might help with the issue. I am not a professional, but, I think it helps to remember that he has been a devoted partner throughout your marriage and you have no valid reasons to suspect cheating . He deserves respect for that.

What is it about yourself that makes you think he would prefer this woman over you? To think your partner prefers to sleep with another woman would indicate that somewhere in that line of thought, you believe that the other woman is somehow better than you. In what way? Is it a trust issue with men? Has a man of importance in your past or present life cheated? (a father cheated on a mother? past partner?) These are issues that should be discussed with a professional to help you figure out what riggers these feelings of jealousy or threats of infidelity.