Is it advisable to marry a woman with whom I had an illicit relationship, then she became righteous, knowing that I love her and cannot forget her?

Repentance and remorse are obligatory for zina (fornication/adultery), which is one of the gravest sins, entailing punishment in the Barzakh (interim period between death and resurrection) and the Hereafter, as well as prescribed legal penalties in this world. It is not permissible to marry a fornicator (male or female) except after sincere repentance. A woman who has committed zina must observe a waiting period of one menstrual cycle (istibra’) before the marriage contract, and if she is pregnant, the contract is not permissible until she gives birth.