Flutherites, can you help me figure out why I am ruining a good relationship over this?

Me and my girlfriend have been together for about a year and a half. We are living together with my 1 year old daughter. Our relationship is great in so many ways, even with all of the shit we have been through… but we both have our issues.

I am a very cynical and free-thinking person. I don’t believe everything that anyone tells me, unless it sounds completely believable and I’m talking to a credible person. I only believe it 100% after I have researched it myself. I don’t let religion or other people’s beliefs influence the way I think. I don’t follow most trends. Basically, I think for myself before I make any decisions.

But my girlfriend is the exact opposite, and it annoys the SHIT out of me.

She believes anything that anyone says to her instantly. She’s come home and told me things such as “My friend ate a burrito from Taco Bell and roach eggs got embedded in her cheek and then baby roaches came out of her cheek!!!1!” or “Did you know you have an orgasm every time you sneeze??”

She also is insistent that everyone in the world is an awesome person with great intentions. Her mother has been using her social security number to open accounts and get credit cards since she was a baby, but she says things like “Someone must have done it to her too, it’s not her fault.” When her cousin (or some other distant relative) called her fat and was a complete asshole to the both of us, she later explained it away that “She was on drugs”, which we all know isn’t true. Even when she was dangling a string in front of the cat’s face, and when the cat reached out to play with it, she flung her hand backward (her fist going right into my face, as I laid in bed next to her watching TV), and when I freaked out she said “But he was scared of it, he thought it was a snake!” ... I’m telling you, she’s not the brightest crayon in the box, to be nice about it. And another example, the other day we were watching that movie The Experiment. She says the black guy is only being mean to the others so that his friend doesn’t get in trouble. I reply, “I think he’s doing it because his whole life his mother has been beating him down and now he has a power complex.” She started to argue with me just as the screen panned to his hard-on showing through his pants.

These things wouldn’t annoy me as much if they didn’t have a direct impact on my life. For example, when my daughter was a few months old, her mother told her “If you throw her in a swimming pool, she will swim. Babies can breathe underwater.” How do I find out about this? All of a sudden, my girlfriend has an interest in taking the baby to my mom’s swimming pool.

Then, more recently, when we are looking into daycares, she goes without me one day, then comes back and says about one of them, “They said that they don’t release kids to the fathers no matter what, so we are safe!” I kindly explain to her that, by law, if he shows proof that the baby is his, since we do not have any specific custody arrangements or a restraining order, he can call the cops and they will be forced to give her over to her dad. She steadily denies my reasoning, her logic being “But that is what the woman said their policy is! That’s what she said! She wouldn’t LIE to me!”

I’m getting to the point where I’m starting to be mean about it. I don’t mean to, but when she comes to me to proudly tell me that her friend saw a unicorn on the way to work, or whatever the fuck she is saying at that moment, I get really, really aggravated. I feel bad the minute that my anger cools off (yes, I do have an anger problem), but once or twice recently in the middle of a fight I’ll tell her to stop believing stupid shit, that her friends are retarded if they are telling her this shit, etc, etc, and I don’t want to be that person.

I really need some advice before I ruin our relationship because of this.

TL;DR: My girlfriend believes every stupid, nonsense thing she hears, and never thinks for herself, and it’s starting to piss me off.