So as I understand it, you're a female who was in a relationship with another woman that already had a live-in partner and she adopted a child with that partner. You were unaware of the fact that she was living with this other person as well. I guess I am surprised that for 1.5 years she successfully hid the other partner from you. I guess never allowing me to come to her home or not being able to spend the night or talk on the phone at night would have raised a red flag for me.
Anyhow, all you can do at this point is move on. She clearly picked the other person if she was living with that person all this time and even adopted a child with her. I think if the other person had any self-esteem, she'd dump your girlfriend as well, as someone like that doesn't deserve to have a relationship.
As others have mentioned, your child can easily get over her. Your baby is not even a year old, she can get attached to you and will eventually forget your girlfriend if she doesn't see her anymore. I don't know why you'd be in love with someone who hurt you so badly and used you for her own selfish pleasure. I'd want to get the heck away from someone like that, and would make every effort to keep my child away from such a selfish lowlife, so your guess about "I'm so in love with her and I don't even know exactly why?" is as good as mine.
As others have also mentioned, regardless of what this meeting may be about, there's nothing to talk about here, she LIED and CHEATED on BOTH you and her live-in girlfriend! Hello? How do you justify that, even if she's Megan Fox or some other hottie??! I'd strongly recommend against this meeting, how can something like this be conducive to anything or even positive? There will probably be a lot of yelling, heated emotions, high blood pressure, cussing, maybe even a punch or 2. It sounds like trouble and nothing good could come out of it.
Save yourself the trouble and drama and the waste of time that this "meeting" would be. If it were me, I'd kick her sorry butt to the curve, block her number, and move on, even if that means staying alone for the next 3-4 years. You can do it! Be strong! You deserve a loyal partner. I know it may not seem that way, but they DO exist. Just be patient. Your child needs you and depends on you, so you must force yourself to stay strong and sane for her sake! Seek therapy or counseling if you're having trouble staying strong and moving on. Nothing wrong with asking for help.