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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If a wife asks her husband to divorce her and he accepts, and she waits until her waiting period expires, then it is permissible for her to marry another man. However, if he does not accept to divorce her, divorce does not take place just because she asks for it.
Divorce is in the hands of the husband so if he does not initiate it, divorce does not take place unless she takes the matter to the Muslim judge and he divorces her from him.
Therefore, if this second husband divorced you or that a Muslim judge divorced you from him, then it is permissible for you to marry whomever you want, whether that is your first husband or else.
For more benefit, please refer to Fataawa 88502 and 90732.
Allaah Knows best.