Can anyone please give me some advice?

I've had trust issues with men for a very long time and my fiance and I keep fighting. He just doesn't get that I'm the type of girl that doesn't like it when another women is in the picture weather he's friends with them or the girl is with a friend of his.
His best friend's girlfriend has been taking my fiance's attention away from me and we're not living together. I trust that he's not sleeping with her, cause she's nasty, really nasty, but the fact that she's constantly there because his boss wants her there at their work shop, I get ignored a lot.
Am I being stupid for freaking out on him a lot about it? Or do I have the right?
My fiance lives at the shop because there has been recent attempts from 17 yrs olds, trying to break in and steal stuff for drugs. He doesn't want to fight with his boss because he needs the job and roof over his head, and that I do understand, but how am I suppose to know and feel that this is going to stop?
He says that it doesn't bother him that I am upset that she's constantly there, it's the fighting with him that pisses him off.
I've tried walking away, stopping to breathe and calm down, but it's no use. It just seems to keep getting out of hand.
We've been together for 3 yrs and I'm afraid that this is it. We might split for good. I already handed him back the ring and he got pissed off and slammed the door in my face.
PLEASE! I need all the help I can get. I'm on the verge of tears. My anxiety pills don't seem to work anymore and as much as people say I need to see a counselor, I'm too stubborn.