Why Is my Girlfriend talking to her ex?

[CollegeStudent]

We have been dating for 9 months now. When we first met, she claimed that they have been on and off for over 10 years. She never talked to him when we first got together. and 4 months later due to a family passing, she told him because he's known the person that 10 years, which is completely understandable. But they have been talking ever since. They text constantly, sometimes until 2 in the morning!!! He calls her, both from his cell and work!! Her dad has a garage, and he is always up at the garage fixing up his truck, and always around. My girlfriend loves animals, so he went out and got a dog, and brought it to the house constantly, asking her if she wants to go to his house to see it! This all goes on when im not around too i should add. He sent her a facebook message when she was over saying "Hey Baby, Last night was awesome, we should deff do it again sometime." (That was false because i was with her the entire time that night). They even went out and had lunch together while I was at work, and she didn't tell me until after everything happened. I was told that it's nothing, and I have nothing to worry about. yet...She's left him, started dating guys and then cheated on that guy, made up excuses to not hang out with that guy, to hang out with this guy. She says that it's different with me, but how can i be so sure? Me and this other guy are on the verge of fighting because he still asks her to go places with him when im not around, insults me, threatens me, and mocks me. She laughs it off like its a big joke, saying "Ohh...Thats just him. hes just trying to get under your skin! hahahaha" What should I do? It's like staying in touch with him is such a big deal, and she says that hes done nothing but treat her like dirt over that 10 years. Every time i tell her it bothers me, SHE gets upset, and "almost gives up". And the best part....HE HAS A FEMALE COMPANION!!! Shouldn't he be texting her instead of MY girlfriend? Shouldn't he be calling her and spending all his time with HER and not MY GIRLFRIEND!? I need some advice, i don't want to leave her, and vice versa, but i don't know how much longer i can deal with this. Of the 9 months we been dating, this has been going on for 5 of the months. PLEASE HELP!!!

[CuriousGeorge]

?

[Brandon]

You need to either get out or honestly sit down and talk to her one on one and tell her that you trust her but you dont trust him, hes a scum and hes trying to break you two up. F*ck him. its alright mate, just have an honest conversation with her explain to her what you explained to us.
Good luck mate.