I read this really long ago so the details might be a bit off, but from what I recall, the main character transmigrated into a mother. It turned out that the boy she thought to be her son was not hers biologically, but she still chose to keep him. And in the original setting, the son did not like the mother (for abuse or abandonment? i forget) but in the story he was worried about being abandoned for not being the biological son.
Could it be: https://www.novelupdates.com/series/delicate-mother-of-a-villain/ ? MC transmigrated as the mother. Later on she found out the boy was not her child, instead the original body had a daughter, but she still acknowledge him as her son. At some point the boy was afraid after he would abandon him for the bio child.