Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
I can very well understand your predicament. You have every reason to be upset over the behavior and lifestyle of your husband.
However, your husband’s behavior cannot be an excuse for you to develop an illicit affair with another man. You cannot rationalize it.
So, the only course of action for you now is to repent and ask forgiveness of Allah.
I don’t know what would make you trust this man who took you out for a date when he was aware that you were already married? What guarantee is there that he would not do the same with another woman after marrying you? So, I would urge you not to get carried away by this infatuation you have developed for this man. Just as you broke the law of God, he also broke the law. And it is good to remember Mosaic wisdom: The Law will break Those who break the Law!
Having said this, let me conclude: If you have no reason to think that your husband is not going to change then you should seek a divorce from him. Then it is up to you to decide whom to marry. You have to make up your mind whether this man you developed a relationship is good or not.
You should repent and continue to pray to God to open your heart to see the truth as truth and falsehood and bless your decision.
Almighty Allah knows best.
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