Why is my girlfriend a crazy bitch?

[Mark]

I've been with my girlfriend for 5 years and we have 2 kids together. Since the beginning of our relationship she's been inconsiderate and selfish. If she doesnt get what she wants or if something doesnt go her way "which isnt very often" she acts completly unreasonable. She starts out by being as demeaning as possible and demanding that I "look her in the eye" as she does this, following me from room to room. She used to hide my keys and wallet so i wouldnt be able to go anywhere and pushing me to the point where i take my anger out on inaniment objects. After this she starts to threaten me by saying shes going to call the cops and acts like some poor abused victim. its really quite comical with her over the top and terrible acting, like shes trying to win the academy award. And just when you think everything has cooled down she starts demanding that i apologize, I know that throwing a remote at the ground or breaking my cell phone in half isnt the best behavior but certainly isnt grounds for me to apologize. I try to give her everything she wants, its very rare that i say no to her, given the consiquences. But find myself always getting turned down if i ask her for help with something. She also has been talking to her ex boyfriend the entire duration of our relationship. She knows how much this bothers me. She'll tell me shes not but then i'll find out that she's either just changed contact name in her phone or created an alternate email or facebook just to talk to him. ive even found a skype account she created where he is her only contact in there. When i bring it up to her she acts so desperatly for my approval to talk to him which is absolutly ridiculous to me. saying there just friends or what ever other kind of excuse she can come up with. I think the only reason she stays with me is that it drives her crazy how willing i am to let her leave. We've been to the point of leaving eachother several times but in the end she fights so hard to stay with me but changes nothing, still acts like a spoiled brat and still secretly talks to her ex. I apoloize for how poorly written this is and this does not even begin to define how crazy and rediculous she is. I dont want to split up my family but i dont know what else to do.