I've always had a good talent for reading people
Yet you couldn't read that she was a nth degree skank who was having sex with her boss for gain while knowing that he was married? Wow... Sorry you didn't pick up on that one.
Why would you be torn about getting a DNA test done? (I hope you've already gotten the STD testing out of the way)Is it because you don't want to know if you are NOT the father? If that's the case then don't have one done and just do what you've always done which is love that child and treat her like she is yours.
Your wife is an issued, opportunist who will do this to you again if the two of you don't get marriage therapy and she gets her own personal therapy in order to figure out why she gets any self-worth she may have (evidently very little) from the sexual attention of other men. She has no boundaries that keep her doing the right thing for herself and for others. She's suddenly not going to get some without professional guidance to help her hone her respect for herself.
Past behaviour is very often a predictor of future behaviour... That is particularly true when there has been zero attempt at self-improvement.
Good luck.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion