I am a married man with a stable family and four children. I have been married for almost 20 years.
Ever since I got married I have been involved in forbidden relations with other women. During intercourse with my wife, many times I feel I am with my wife and another woman as well because this enables me to get more aroused. I also share this feeling by talking about it to my wife during the act.
I am a Muslim who observes his religion fully and is involved in many activities helping other Muslim brothers and the cause of Islam.
My question is that I am now seriously in love with another woman and am thinking of having her as a second wife.
I have spoke to my wife about this issue because I believe that this second marriage will get me away from the forbidden relationships I have had all along.
I love my wife and family very much and do not want to affect this stable family relationship.
The problem stopping me is that my wife does not agree to this second marriage and strongly opposes it.
My wife is a good wife and a good mother. Alhamdu Lillah I have no complaints about her, but the issue of my wrong doing is also continuously disturbing me.
Please advise me what to do as I strongly feel that if I don't get married to this girl I love I will continue to have other forbidden relations.

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.

Having any relations with a marriageable woman out of the boundary of marriage is forbidden. See Fatwa 81356.

We have said in Fatwa 89671 "First of all you have to sincerely repent to Allaah for that great major sin that you had committed. Indeed, adultery is one of the greatest major sins and Allaah warned us against it in His book saying (which means): {And come not near the unlawful sexual intercourse. Verily it is a fahishah (i.e. anything that transgresses its limits; a great sin) and an evil way (that leads one to Hell) unless Allaah forgives him/her.}[Quran 17:32]. Therefore, whoever has committed such a sin has to sincerely repent to Allaah fulfilling the conditions of a true repentance; for details refer to Fataawa: 87903 and 83611.

Besides, you are required to conceal your sin and not reveal it to anyone, as the Prophet said: "Avoid such filthy things which Allaah has forbidden (sins), and whoever was involved in any of them, has to conceal himself and repent to Allaah because if one reveals his sin to us, we have to apply the punishment on him." [Al-Haakim]" [End of Fatwa quotation]

Imaging another woman during intercourse with one's wife is forbidden as we mentioned in Fatwa 86448.

There are some conditions to marry another wife as mentioned in Fatwa 81469. If you are able to fulfill them you are allowed to marry even without the consent of your first wife as mentioned in Fatwa 87921. Your wife does not have the right to stop you from taking a second wife see Fataawa 86514 and 89883. 

Allaah Knows best.