Adam Clarke on Genesis 38:26

She hath been more righteous than I - It is probable that Tamar was influenced by no other motive than that which was common to all the Israelitish women, the desire to have children who might be heirs of the promise made to Abraham, etc. And as Judah had obliged her to continue in her widowhood under the promise of giving her his son Shelah when he should be of age, consequently his refusing or delaying to accomplish this promise was a breach of truth, and an injury done to Tamar.