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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is forbidden in Islam for a man to establish an affectionate relationship with a marriageable woman, and such a relationship with a married woman is even more sinful and more serious. If both the man and the woman know that this is forbidden and still do it, then this is an even greater sin. So, you should take the initiative to repent and not be lured by the whispers of the devil that repenting is too late because it is never too late.
If you continue this forbidden relationship with this woman, you are breaking her relation with her husband, and this is yet another grave sin. The Prophet, sallallaahu ‘alayhi wa sallam said: "He is not one of us he who spoils the relationship between a woman and her husband." [Abu-Daawood] Some scholars are of the view that the marriage of a man with a woman for whom he was the cause in spoiling the relationship with her husband is not valid. However, the majority of the jurists are of the view that such a marriage is valid but the person is nonetheless sinful. Nonetheless, you should be careful and you should stop this relationship.
Pious and righteous people should endeavour to reconcile between this woman and her husband, as there is good in reconciling, and this is something possible. Indeed there are many spouses whose relationship was very bad but ultimately, they reconciled and had a good marital relationship.
You should rather think about another pious woman for marriage as there are so many of them, and it might be that you will succeed in this. Allaah Says (what means): {… and it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allaah knows but you do not know.} [Quran 2:216]
Finally, it should be noted that if divorce takes place between that woman and her husband without you causing this divorce, then there is no harm, Allaah willing, in you marrying her.
Allaah Knows best.