So far what I'm seeing is....
Once she started attending school and you weren't meeting her high expectations she developed all this contempt for you and told herself "he's not a good bread winner.... I can do better.... I deserve better".
In December you saw the warning signs of an EA and when confronted she brought up divorce. I hate o tell you this but there affair would have been going on for about 6+ months at this point and she favored him more over you.
Don't try to compare yourself to him in any way because he's been filling her head with a load of crap about how much better he is than you and would be better for her.... it's all a lie and a relationship built on avoiding conflict. All that conflict has to go somewhere and she's placing it on you.
She may have been agreeing to go to counseling with you to "work on the marriage" but she was escaping out of town and sleeping with her ex. I recognize these lies about "just a friend", "stay out of my business", and "you're too controlling" right off the bat.
I can still tell you there is hope because if nothing else ..... she had a baby with you and you're going to have at least some contact with her in the future. But for now you have no other option than to take yourself out of the running and agree with her feelings.
You need to apologize to her for being so controlling in your marriage and keeping her from finding someone more deserving.... I know it's not what you want to say but it's what she needs to hear if you want to take away the stick she's beating you with.
After that prove that you're not controlling by giving up and not trying to work on your marriage anymore, avoiding contact with her until the affair is over. Every time you try to change her mind about divorce she's just going to keep attacking you and if possible restrict you from seeing your child to keep you away from her.
You've got a divorce meeting (mediation?) coming up, great! Show up with a smile on your face and act like it's no big deal because really it's not.... just a bunch of papers that need to be filed, but it doesn't take away the good memories and underlying feelings you have for each other.
You must agree with her as much as possible or come to an agreement that won't financially screw you over. If she demands full custody five it to her
How long do you think she can last balancing motherhood, a secret affair that ended in divorce, lying to friends, family, and coworkers about why she divorced you.... getting tripped up along the way, the depression soon to follow divorce (and I've seen a lot of women come out of mediation in tears an hour after giving high fives to each other), and the loneliness that comes from cutting off close friends and family to be with the OM living without judgement for the horrible home-wrecking decision she's made?
You know that OM is jealous of you and won't be a good father or will at least fake interest to get sex.... that's what this is mainly about. Think about it.... you know exactly how she can be when she's not trying to be nice, he has yet to see this. He has yet to see the grumpy, disgusting, mean, PMSey, b!tch you grey to know and love.... add waking up every two hours to a baby crying it's head off to be fed, stress from work, don't touch me like that.... sometime after the fun wears off, and "Now I remember why I broke up with you [ex-boyfriend], you haven't changed a bit".... and you've got yourself a recipe for a relationship disaster.
So sit back, relax, and let him deal with all the crap you would have to deal with just waiting this this thing out and working on yourself. Yes you'll miss out on some things, and that sucks! But think of child support as your get out of bullsh!t pass. You're paying her to feed and clothe your child. All the house work, stress, depression, fights with the bf, and general more of the same she thought she could escape from come on her tab for both of them to pick up