What do I do?

Keep online and physical copy of the screenshots in several places.

My wife says that she was just caught up and didn't mean what she said, but it hurt sooo much.

If you believe her I have a bridge in brooklyn I can sell you, are you interested ?

Read this : http://talkaboutmarriage.com/coping-infidelity/32002-welcome-tam-cwi-newbies-please-read.html Especially the part about reconciliation. And as you are there read other people stories.

I advice you to visit a lawyer for know your right, even if you don't plan to divorce.
Because more serious and decided you will look like, less she will take you for a joke, and she already takes you for a joke.

DNA test your daughter, even if she looks like you. It will send a powerful message to your wife "I don't trust you anymore".

Get STD tested.
As she is not an experienced in lesbian sex, she certainly doesn't know how to protect herself.

Print this and make her read it : http://affaircare.com/articles/understanding-your-loyal-spouse/

Now she must send a nocontact letter to this friend and get rid of her, no contact with her, if she break the nocontact you serve her.
Does she continues to work with her ? If yes, she needs a new job yesterday.
Then she must expose herself to her family, your family, her and your friends, if she refuses, you serve her. Why ? Self preservation, simply.
Like this she can't said you was an awful and abusive husband if you take the road of the divorce.

Ask her a full timeline of the affair. Don't need gory details, just how, when, where.
Then tell her she will pass a polygraph, it's a bluff move.
If she refuses, you will know she lies to you, if she accepts, plan it. Be ready to see her change her opinions about it.

You need to go to IC, her too, if she refuses, refuse to work on the marriage, and serve her. Then MC can happens.

I know they had called it off before I found out, but how can I ever trust her again?

Do you realize she never planned to talk to you about it ?
You should thanks the Lords of Kobol.

You can maybe rebuild a different trust but your marriage is gone, the woman your married too.

Now the things you should always keep in mind :

You will never nice back a cheater.

You must be ready to love your marriage for save it.

A real reconciliation takes between 2 to 5 years. It takes more to get back to something normal.