I've been married for less than ten years. She left me a few years ago for the guy that took her virginity. I never divorced her, hoping she would come back. I honestly didn't know where she was or if she was alive or anything.
She called my mom over a year later to tell her she wanted to go to rehab and had been using meth. My mom drove across two states to take her to rehab, but she never showed. A half a year later she called again and said she wanted to go to rehab, so my mom made all the arrangements for her to go to a Christian-based rehab. She actually went this time and on the long drive back told my mom she was pregnant.
I went to the state she was in after she had her baby and visited her and the baby. It was so painful and so good. I went back home. I could only visit every other month according to the program she was in and less than that because I was so far away in another state.
I ended up feeling more and more hurt that she had done this to me over two years of not knowing. It was so random and unexpected for me and my family and her family. She and I were close to God, and it was just absolutely out of sorts.
I ended up sleeping with a girl, dating her, and then left her for another girl. I told my wife and told her it was over, that too much damage had been done. I stayed with this girl for almost a year. We fought all the time. She is bipolar and has a very long history of burning every bridge she has come to. I actually proposed to her, moved in with her and was going to get a divorce and remarry this girl. She started arguments all the time, would trash the house, and just, overall, was terrible to be around. Everyone who knew her warned me about her. I told her I didn't want to be with her anymore and started feeling convicted to return to my wife. The day after I broke up with her she tells me she is pregnant.
My wife is still in the program as a leader in her second year by choice. She has her daughter and is doing good. The girl I got pregnant is in jail for domestic abuse, criminal mischief, and trying to attack another girl she thought I was sleeping with.
I'm not sure how to handle this.
All of these stem from both you and your wife not obeying God. Your wife committed adultery and became a drug addict. You, in revenge, became an adulterer. Both of you, I suspect, tried to fix problems with sin, and that never works.
First off, I would not count the girl you have been having sex with as being pregnant until there is some solid evidence that she is pregnant and that it is your child. The odds are that she is pregnant and it is your child, but when you deal with people like her, lies come too easily. If it turns out to be your child, then you need to get custody of the child, if possible, or see that the child is adopted out to a good family. If this woman insists on keeping the child, then arrange with the state to make child support payments, but don't directly deal with the mother unless it is about picking up and dropping off your child. You have no right to marry this girl that you got pregnant (Matthew 5:32; 19:9).
You and your wife are best off reconciling. You both need to put effort into it and expect that it won't go smoothly. Get to know your daughter. She may not be yours biologically, but she is legally your daughter. She will need a real dad, and you can be that man.