You and I have had our differences, but there's an aspect to your situation that hasn't really been addressed in this thread. Assuming that the baby is yours, the fact that the mother is a "colleague" adds complexity to whatever you do to try to regain the trust of your long term partner. My best guess is that you periodically need to work late, as does your colleague. If you're both litigators, that really can't be avoided.
If saving the relationship with your long term partner really is important to you, you might want to explore ways to manage your career so that you and the baby momma work in different places. Maybe that's not a realistic option, but your long term partner is more likely to believe that she can trust you in the future if your conduct (not just your words) shows that you will do whatever it takes to end any relationship with the baby momma.
If your long term partner was my sister, I'd tell her that if you weren't willing to do whatever it took to get the baby momma out of your professional life, she should leave you.
You asked for it unvarnished (which I respect). So there it is.