"I feel my relationship with my girlfriend would be so much better if her daughter wasn't in the picture."

Well that's not feasible. So that being said, I suggest you find a nice woman without children. The co-sleeping is simply an underlying issue of a much bigger problem.

My advice will be unpopular yet I am going to give it all the same. Get your GF to start a 10 day program to move the child in to her own room. 1 night in the bed, next night a mattress on the floor by GF's side of the bed, 2 nights later mattress by the door way. You get the idea and move the child to her own room.

I cannot remember the name of the show I watched, it was British. Parents with difficult youngsters, mostly bad parenting were taught how to get their children to sleep in their own rooms, stop tantrums and so forth. This was a method they used which seemed most successful.

I co slept with my eldest. When I moved in with my now husband I helped my daughter to sleep in her own room. She was 4 years old soon to be 5. It is easier than people make out. Shouting, punishing and so on are just not needed. I started slowly with putting her to bed and reading her a night time story. I lay with her for 10 minutes then lights out and door 80% closed.

Then I started to sit next to the bed, then by the door way. If she got up I repeated the same steps to put her back to bed. I did not allow her back in mine. My eldest has no side effects from this. It was stress free and calm. Your GF needs to be the person who does this. No matter how many times the child comes to the bed it's a rinse and repeat effort. It takes 3-5 days best case and 6-10 worst case but you can never falter.

I didn't waste my time with night lights or favored toys or replacements for me. It was a firm resolve as I wanted our bedroom for us. There are treats where all the kids were allowed to come get in to bed with us on a Sunday morning. We read, played, told silly stories. All the kids were included. My daughter, SS and my husbands ex SS.

" I feel my relationship with my girlfriend would be so much better if her daughter wasn't in the picture. "

This feeling will never get better, it will intensify, until you become bitter. You think a six year old girl is difficult? This is NOTHING. Teenage girls are complete pains in the ass, even when they're your own.

If you don't have kids with this woman, get out! And whatever you do, don't get her pregnant or you will be stuck!

She can't keep her kid out of the room? Malarkey! She can lock the bedroom door. I agree with the plan to remove the child from the bed by one night in bed, one night on floor in sleeping bag (I wouldn't bring a mattress in and make it comfy), closer to door, hallway, then her own bed, then lock the door at night. Your gf has to do the work since you're night shift. Get a key for the bedroom and have your gf lock the door. I made it very clear in my relationship that his daughters place at night was in her own bed and his place was in bed with me, no exceptions. She isn't even allowed in our bedroom, ever. She stole from us at 8 and that ended her ability to go wherever she wanted in the house, when one of us isn't around. She's permitted in the bathroom, her bedroom, and the kitchen. We don't even allow her in the living room by herself, unless one of us is close by, because she rents movies without asking.

Your gf should fully respect you wanting some control over your sleep setting and having a say in what goes on in your home, it is normal to need some control of these things. I suspect she likes the closeness, since you aren't around at night, but that's no excuse if you're uncomfortable with it, as its your bed and home too. She's using her child to fill the place of a lover and that's unhealthy for everyone. She needs to sit and think about why she's okay with her kid sleeping with her, what it does for her, and if that's healthy for her or to put that on a child, because a child's place isn't to comfort mommy and this child needs to learn to self soothe and gain some independence. Boy you're going to have a piece of work SD to deal with if mom keeps this up, if there are other codependency issues between mom and daughter. It will be hard for mom to put a stop to this behavior, and other codependent behaviors if they exist, but it's vital to her daughters well being and to your gf's as well. Good luck.

This child is 6 years old, that's still very young. And while I agree that it's not helping your relationship and needs to be sorted, I'm not sure it will be just by banning her from your room, issuing ultimatums or locking the door.
You GF needs to deal with the issues behind her wanting to do this, you say she doesn't see her father, maybe if her father is out of the picture she needs reassurance that her mum isn't going to disappear either?
6 is very young, they don't process things the same way as we do and while it's understandable that you feel the aggrieved party here, there is no right and wrong and you're the adult here.
I think you need to talk to your girlfriend calmly about this, without it sounding like you're accusing her of anything or issuing ultimatums, parenting is a very sensitive issue and it's easy to feel like someone is having a go at you. Maybe your GF could take her daughter back to her own bed but stay with her a little while, and make this time shorter and shorter every few days? It's understandable that her daughter comes first in this but she also needs to work on her relationship with you, and you need some sleep.

I have to say though, this phase will pass, but it could probably do with a helping hand.

Good luck.