My pennorth worth, as one who has been insanely jealous, is that it is a matter of low self-esteem. Basically you don't think you deserve the person you are with and set out to prove it. I don't think it has anything to do with whether that person is trustworthy or not, it's to do with yourself. When you accept the truism that all men are bastards and are jolly lucky to be part of your life, you will be on the road to recovery. BTW, a very good book to work through re CBT is "Feeling Good" by Dr. David D. Burns. It is aimed at depression which is basically about negative, warped thoughts so can be used for other areas too.