when one lives with one for 6 years , and he has not married you all bets are that he does not intend to, also it may hurt, but it looks like he has found someone else, so let him go and do not embarras your self begging for his love he will just run faster i do feel for you, and you may find someone else when you quit chasing him luck jo
Sorry, but this does not sound good to me. If he really cared about being with you, he would tell this girl to sling her hook. No way does any guy who really wants to be with some girl, will put up with a flat-mate that refuses to let his girlfriend stay over, even if that was another guy! He is spending more time with her than he is with you - even if she didn't have a history of being "obsessed with him" this would be worrying. This is a 1-bedroom flat - where is she sleeping?
You need to find out where his loyalties are. You've been together for 6 years, that's a fair amount of time, either that should count for something or he's lost interest. You need to find out which. If he is not prepared to tell this girl to find somewhere else to live, and focus his attentions on you, it would sound like he's not that bothered about maintaining this relationship.
When issuing ultimatums, it may help to show a bit of give and take, that you are not being unreasonable and are prepared to contribute to the solution. So you should say that you are not insisting the girl move out tomorrow, that he should give her enough time to find somewhere else to move to. I don't know what your financial situation is, but you should offer to contribute to household costs - at the very least cover your own costs (half the food bill etc.), ideally a share of rent/electricity/'phone bills too. If he doesn't go for this deal, I don't think you've got much of a relationship left.
he tells you he wants you and wants you to move in with him, but when you get there he has a roommate??? is that right?
i see alternatives being that you stay where you were, not living with him, while continuing to date him if you MUST have him or better still
leave him and get with guy that doesn't share 1 bedroom apartments with ladies