Hi Nik79, welcome to SI, though I'm sorry you had reason to be here. Even if it seems everything else has gone to hell, you've at least got SI in your corner now.
Perhaps it wasn't a time of weakness but rather a time of resolve to get answers when you went to her phone. You felt in your gut there was something going on and it wasn't like cheaters take out a billboard to keep you in the know. Rather, infidelity thrives in secrecy.
In confronting, you must be VERY sure not to reveal how you got your information. Don't know if you've been reading here much; I see you registered last month so hopefully you have. However, if not or you've missed it, a lot of times affairs go underground. The WS now knows you're onto them and they do the minimum to keep you on the line while simultaneously going dark ops with their communication.
If you haven't already made copies of the information you saw on the phone, you should consider doing so. Sometimes the denials the BS hears are so vehement and so manipulative as to make you question whether you "really" read what you think you read, saw what you think you saw, etc.
You may also want to consider how you will maintain access to information should your WS profess to be penitent...because sadly too often saying is easier than changing the behavior.
I know my act was bad but I want to confront her just as she leaves for her trip. I want her to know that if she still goes through with this weekend, she will have to do it with me knowing on her mind.
I covered the "act was bad" part initially; the second part, about "me knowing on her mind"....gently, she's known she's married right along.
Stopping the behavior must come from remorse and a desire to change, not from whether or not you know.
You didn't describe much about the upcoming trip. How long has it been planned? How far away is she going? Who's the friend?
Depending on your individual situation, and details I don't know, I might be inclined to find a reason either she can't go or that you CAN go. Or figure a way to spin the information you have to where you won't reveal your source, confront, and say you're not comfortable with her going.
First and foremost, where were you at with the marriage before this? Were you contemplating divorce? Is reconciliation even something you want to consider, or ?
Good luck to you. Have you read the Tactical Primer in JFO yet? http://www.survivinginfidelity.com/forums.asp?tid=235051
Do so if you haven't.
*****
EDITED TO ADD:
I see several posts added since I started writing mine (I'm slow!) ~
I'm with gonnabe2016 & jjct. There's been an active affair, it appears; you've confronted once, and she's continued. Right now you've got too many people in your marriage.
[This message edited by sad12008 at 10:25 AM, May 11th (Saturday)]