Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister, thank you for trusting us and for your question. Indeed, we commend your interest to seek advice from scholars on such critical situation.
Zina (adultery) is a grave sin that Allah prohibited categorically and set severe punishment for. Your husband’s big mistake never justifies it in any way. So, you need to fear Allah, cut your off relation with this man and make a sincere repentance immediately.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
You have committed the most grievous sin, adultery. The consequences are indeed most serious.
This is not to minimize the responsibility of your husband in contributing to it. He is indeed guilty. However, this in no way should make you rationalize and justify your action. If your husband could not satisfy you, and you don’t think he will ever be, then you have the choice of asking for divorce.
Adultery is not the solution. Therefore, I would urge you to repent to Allah and cut off your relations with this man. And see if your husband can be treated. If he cannot be treated, then you are free to divorce him and marry someone else.