You won't get any criticism from me! My daughter was born with a primary sleep disorder, and sleep has always been an issue. Exhaustion can (in my unprofessional opinion) be one of the most powerful and debilitating causes of stress - whether it's personal, health, marriage and/or home It seems like it would be so simple, doesn't it? Your child should sleep at night like the billions of other kids around the planet, yet she doesn't. I know how frustrating and difficult this can be, for everyone.
There are a few things you might try, for your daughter that are fairly simple:
A weighted blanket. Just type that into Google and you'll see many available. It can provide calm and a sense of security. They're not super cheap, but what price can you put on sleep?
A white noise machine. They're available online and they cut out noises in the house that can disturb a light sleeper.
A scented cloth. I bought a small square of very soft fabric in a pattern that my daughter liked at a fabric store (you can also use a handkerchief or face cloth), and I lightly sprayed it with my favorite perfume. I put it under her pillow and told her to smell it when she felt afraid or lonely.
Take her to a store and let her choose a night light, a lamp, a pillow or new bedspread, things to make her room her own. If she wants superheroes, fine. If she wants a weird ugly shade of green that she loves, fine. If your home is tastefully decorated and she wants glittery unicorns, fine. Help her re-decorate a little (as your budget allows) but remind her that she's creating her space, not a space for you and her. You have your bedroom, she has hers, her brother has his.
Now, as far as you're concerned, you seem to already know that consistency and staying with a plan are crucial. But I know it's hard. Our daughter's psychologist warned us that when helping a child to overcome something (sleep problems, behavior problems, etc), when the parent has a plan in place and begins to implement it, the child's behaviors will often get worse in response. It's expected (by the professional, not by the parents). Often the parent will say "well, that didn't work, so I stopped trying that". But actually it's working, and it's the child's natural response to the correction (like when a toddler's tantrum gets louder if a parent ignores the tantrum).
So, I suggest that you sit down with your daughter and talk with her. You can help her come up with a plan for night time. You could try drawing a calendar for her, and write down how the night will go. Make sure she doesn't have access to electronics of any sort for an hour before bed (the blue light that all electronics have can disrupt sleep patterns). Promise her you'll read to her or with her before bed. And try not to let her realize how desperate you are for sleep - you want to make this about independence for her, and not relief for you. You might try talking with her about things she'll be able to do when she gets a little older (go to a movie with some girl friends, stay up a little later, have a sleepover or go to one, babysitting, having a phone someday, etc). And you can remind her that those are for kids who have achieved some independence, and that includes not sleeping with mommy. But stay calm, and don't let on about how disruptive her sleep issues are to your marriage and home and health and mental well-being.
Then come up with a plan together about what to do if she calls out for you. Tell her you'll appear in the doorway, and wave or blow a kiss, but you won't be coming in to the room any more, and she's not to come to yours. Remind her that all people need sleep and she may not disturb the sleep of the rest of the family. If she needs to turn on her night light and read, ok, as long as she's quiet. If you must, set a goal for her. 7 nights without calling for you or coming out of her room or making a loud scene in her room, and she and you will do something grown-up and fun (a manicure, a movie, etc).
Then tell her that you love her and this is not a punishment, but part of growing up.
And then, plan on a few rough nights. But you have to stick with it like an Oscar-worthy performance. Don't give in, don't cry, don't fret, don't yell, don't even sigh. Remember, if she cries louder and comes to you even MORE than usual, it's working. Stay calm as a rock.
If she won't agree to a plan, and if she won't cooperate at all, it may be time for a specialist to give you some help. A doctor, a sleep specialist, a psychologist...there is no shame in asking for help.
It may not help at all to ask her what is bothering her, why she is afraid or reluctant to sleep alone. She may not be able to verbalize her feelings (assuming she has not experienced any trauma related to sleep or night time). Begging her to tell you what is going on may just frustrate both of you. If there is no recognizable reason that you can think of why she dreads sleeping by herself in her room, then you will have to get a little tough, and get consistent.