Some issues are harder to deal with than the simple question of who the baby's father is, and a woman can sometimes transfer her anxiety onto the paternity question and think that is what is keeping her awake at night. If the underlying worry is something else, solving questions about paternity won't help.
Women who just can't relax about a DNA test might have a number of different anxieties. It could be guilt over how she behaved, especially if cheating was involved, or a concern that the fact she did it means she is not happy in her relationship. It could be catastrophizing, such as, if he finds out I cheated the world will explode. It could be worry they won't be a good parent, or wishing the other guy was the dad and being ashamed of it, or resentment of her husband, or fear of childbirth, or fearing divine punishment for being a single mother, or not really wanting to be a mom, or worry about money, or something else entirely. Worries like that are not easy to control, especially if the problem is shame or guilt. (This latter kind of lingering angst often is a specialty of women who have never had to face up to having done something 'wrong' that can't be taken back, glossed, or fixed.)
When such a feeling drives anxiety, the brain hates that it doesn't feel solvable. A person's mind in this kind of existential stress often lets the anxiety settle on something cut-and-dried that to the brain feels more controllable. (Like obsessing over what if the tests are wrong and the father is someone else, what-if what-if what-if.) But unfortunately, no amount of rational answers about paternity will solve worries that aren't actually, deep down, about paternity. Talk to a counselor about your bigger fears, and your fears over paternity will leave.
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