It would be a good idea to talk with a counselor, the way a person should over any intrusive thoughts that keep them from a normal happy life. In the DNA/Paternity community we see all the time that women with guilty consciences over sex with the wrong guy often settle on obsessing over dates (even in the face of all facts and evidence), because it is easier to brood over that, than it is to accept that one did something that is against one's values. Often no amount of answers and reassurance over the dates stop the anxiousness because that is not really the thing that is bothering the woman. Deep down in her heart she knows the tests and scans are right and knows who the baby's father is, but the anxiety has to settle on something, and she would rather be anxious about dates than feel like a bad person for having had a one-night stand. Work on the reason you feel anxious, the guilt, not on a false target like an unrealistic worry about dates.