Has a girl ever said this to you?
 

"Its not you I don't trust, its everyone else".

Have you ever heard this? What do you think or how do you respond?

Was she darting her eyes back and forth while rocking back and forth as she said that?

It's just some fluff talk from her I wouldn't even hear it. Just drive the conversation where you want it to go and ignore her crazy talk.

usually a gf would say that because you are going out on a night out with the lads and shes not going to be there and feels insecure about losing you
she thinks that some uber chick will steal you from her

i came here to say exactly what reflex said.

except he forgot to mention the doormen he was givin pocket wanks to...

LOL, yeah, I have heard it from a couple girlfriends.

Its quite annoying because no matter what I say, she's still guna think I'll end up cheating. How do ya stop a girl thinking that?

Don't sweat it. At the end of the day its not down to you to change her thoughts. If its brought up state the truth and don't go banging other chicks.

Simple.

Yeah Darood you're right. I looked into it too much!!

just make sure you dont fall into the trap of letting her insecurities dictate wether you see your friends and enjoy yourself.
also ensure if her insecurities become more than a passing comment and instead turn into a source of friction/negativity that you make her aware that this is unnacceptable

Just to echo what Reflex said! Don't even bother arguing about it, just tell her you are going out, you won't cheat and that you trust her to be faithful in the same way you are to her. It lets her know that you see it as her not trusting you fully over something insignificant.