My girlfriend and I entered into a poly relationship with one of our best friends. I said no at first because I didn't want my relationships with either of them to change, but I eventually said yes because I wanted to keep everyone happy, and at least try even if I still felt uncertain. It's only been a day, but I spent time with our friend today, and although I do love her, I feel it's not romantic love, but I don't want to end it for fear of hurting them both. I don't know what to do.
To "keep everyone happy" is NOT a good reason to do anything. If you don't want to do something, someone else's feelings don't get to override your boundaries or even your preferences.
It sounds like you don't want to date this person. Don't date them, then. Even if it will hurt their feelings or upset your girlfriend. Staying in a relationship you don't want to be in for the sake of someone else is unfair to you and the other people.
Be honest - tell your girlfriend that you agreed to try it more to end the conversation and keep her happy than because you actually wanted to. Say that you've thought about it, tried it out (albeit briefly), and you've realized that you just don't want this arrangement.
Think through your best case scenario so you can have a conversation with a purpose. Do you not want to be part of any polyamorous arrangement, or are you okay with your girlfriend dating this friend and you keeping your relationship platonic? What are your feelings, needs, and desires here? Try to have clarity on what you're asking your girlfriend for, then ask for that.
Once you ask her for what you want (to re-close the relationship; for you to be left out of the new relationship, etc.) then she is free to say "no." If that happens, you are not obligated to stay in a relationship situation that isn't working for you, even if you holding your own boundaries makes other people hurt or upset. It is not your job or obligation to keep everyone happy!