Welcome and I'm sorry you're going through this. Ariakas had a good point when he said that you just may not be poly. You can logically understand it, but it still doesn't resonate with you. THAT'S OK!!!! This is a lifestyle that's not for everyone. Now, with that said, let's move on to your gf.
She herself may not truly understand what it means to be poly. I would think that should give you as much time as you needed to process through your feelings and emotions. I don't really like that she's making dates BEFORE she says anything to you. Hell if I start chatting with a woman ONLINE, I make sure that I let the girls know. Honest and communication are two very important foundations in this lifestyle. I'm not sure she's taking the proper steps to assuage your fears. Maybe you should suggest to her that you both research polyamory together. Explore the possibilities together as a couple.
Now, back to you. You said that you ruined a few conversations. Again, not TOTALLY your fault. You're human and have lots of emotions happening right now. Unfortunately this may be why your gf doesn't feel comfortable coming to you before hand. Maybe she feels that if you don't know about it, it won't bother you. Not in her head, so I can't say for sure, just trying to go off of experience.
Good luck and keep us posted.
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!