Why do girls get mad over the smallest things?

Okay, so me and my girlfriend LOVE to play video games. She's plays less than me and she kicks my *** in a lot of games I'm good at. We played Halo 3 from like 10:00 PM to 6:00 AM. Now she works a lot and has no time, and when she does, she wants to play a game different from what I play. When she wants to join in on any game I'm playing, I ask her nicely to wait, until I finish a match, then she gets mad. I recently got her to play Black OPS, and of course, she kicks my ***. She says she sucks at every game she plays and now shes mad because she feels left out. She has a thing for perfection, if she can't do it once, she automatically gives. I've got MLG friends who've seen her play and call her a beast. She plays Marvel VS Capcom 3, and kicks all my friends you are pro's. Her excuse is "I don't know what I'm doing, I just button mash". She buys a lot of games now, knowing that she almost has no time for play them and then complains about not being able to play them. I love this girl to death, but this complaining needs to stop. I can't sleep well because she gets mad at me. I ordered an EASYCAP device so she can play the PS3 or XBOX 360 on her laptop. She got happy so she can finish her games finally, but until then, she wants to play a game with me that she hasn't touched in a long time and expects to be the best. Everyone has a hard time trying to adjust back into their old games like Halo. Now we had this stupid argument on what game she's could play is good at but of course, she says she sucks while in fact she does not. Now shes in bed sighing really loud trying to get my attention, and when I give it to her she pretends to sleep. I want to know what I can do to calm her down without me getting angry at her or her getting angry at me. What could I do to end this nonsense?