re: What would you do if you found out your baby wasn't yours?

are you the person and are you actually thinking of using our advice?

too many variables for this medium.

do you now know who the actual father is?

Do you and the Mrs not plan to sue the actual father for child support? That is my first response to your actual question.
I would hound him to the ends of the earth. He would pay and pay and pay and pay.

Tricky thing is whether a court would require you to let him have visitation rights if you make him pay child support.
OT legal team?

The average marriage lasts about 7 years.
There is a chance you will not be there when the kid starts kindergarten. I would cultivate this person into expecting to be the dad in fact, have him on the back burner, let him meet the kid, and you three have some time together, some pitch and catch, go fishing.