If she has the bladder control to hold it ALL DAY, then there is no medical reason that she can't be potty trained.
There is a website you could goggle, Potty training in one day. The concepts are fabulous. and you wouldn't need to by the information or extras,
after age 3, Potty training has a ton more to do with what the parent does than the child being ready. I'm not saying that critically, but to give you hope that You can help her get this. I would totally back off for 40 days, give her a whole month of diapers- ( is she in preschool wearing pullups?) Then, when you can, spend at least 3 days straight at home with her working on this, A week would be even better.
Gather up a bunch of potty training books and movies,
Tell her calmly that you are going to help her learn to use the potty like all her big kid friends, and like mommy and daddy and auntie and grandma, Barbie, Dora, Strawberry short cake etc.
Give her tons and tons and tons of her favorite liquids. Remind her to drink if she gets busy playing.
tell her that you will be taking her potty every 20 mins and she gets a prize ( I would step it up from just stickers and m & m's to something pretty impressive for her) just for sitting. SHe DOES NOT HAVe TO pee, but she does have to sit there while you read her 2 story books or sing 4 songs with her(The goal would be sitting for 30 secs, then increasing it just a second or two each time no more than 2 1/2 mins unless she is happy).
read the books, then let her go play, give her lots of drinks and in 20 mins bring her back to the potty and do it all over again. if she screams calmly carry her to the potty and sit her on it--you are the adult and she needs to listen. AFter 30 seconds or 2 mins what ever you did the last time, let her go and give her lots to drink and bring her back in 20
Eventually she will Pee and then you need to pull out the Balloons and Streamers and cracker jacks and the marching band and call daddy and everyone you know, Call Dora or Hannah Montana and tell them the good news. Praise Praise praise, reward reward reward.
then give her lots to drink, and bring her back in 20 mins,praise her praise her praise her for sitting Do this all day and the next day and the day after that and she will have it.
At some point she will balk, and probably have an accident, calmly give her a towel and make her clean it up (of course you would really clean it up but she at least has to think she is responsible for cleaning it up) then make her put the soiled clothes in the hamper, get new undies and get dressed, Then offer her more to drink and repeat. You can't get angry though, that just sets everyone back and the whole thing takes longer. Find someone to support you on this and take turns spelling each other, but you both have to be consistent.
I did this with my daughter at 18 mons and she got it in 2 days.
A few cute thing you can do is have her dolly go pee, slight of hand pour a little food colored water in the potty
Blowing bubbles often helps them relax their muscles so that is a fun thing to do while sitting on the potty. Maybe blow on a pin wheel, anything fun and distracting.
But once you start you have to be consistent about taking her every 20 mins. Ands she will regress and have accidents but you stay consistent and don't ever go back to the pull ups. For this to really work you just have to basically stay home for a few days and work on it. With youonger kids i would let them do pull ups at night but if she can hold it that long during the day she could easily flip that schedule around and hold it at night, Get plastic sheet covers, and prepare to do a lot of laundry, have several clean sheets and jammies ready to go, and i would go so far as to wake her up every hour and set her on the potty. I think if you put her in pull ups at night, she will just continue to do what she is already doing, and that isn't good.
Almost forgot, Kmart sells the plastic underpants made by Gerber, they are great, the child feels wet but usually the plastic outer part protects clothes and furniture. Those i might use at night. but i would still wake her up.
Good Luck, and let the pressure off yourself, sometimes we just get in bad cycles with our kids and if You are committed you can break them, what is happeing now is working for your daughter so why would she want to change it. You have to be the one to help her.
Plan a HUGE treat for yourself when this is over because it is hard work.
Let us know how it goes.