All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
You are not obliged to look for this woman in order to give her back what she gifted you, because either she has money earned from permissible and forbidden income, so it is permissible for you to accept a gift from her unless it is purely the forbidden money that she gifted you, or that she only has money earned from forbidden sources, and Islamicly forbidden money is not considered her property. So you should spend it in charitable fields with the intention of getting rid of it and not with the intention of giving alms as Allaah is Tayyib (which implies purity) and He accepts only that which is pure. For more benefit on getting rid of forbidden money, please refer to Fataawa 83783 and 85315.
Finally, you should know that if you have not yet reached the age of puberty when you sinned, then Allaah willing, you are not held accountable for it, as the Prophet said: "The pen is lifted (stopped from writing the deeds) from three kinds of people: a sleeping person until he wakes up, a mentally-handicapped or insane person till he can reason, and a child till he reaches the age of puberty." [An-Nasaa'i]
Allaah Knows best.