Are there any barrel racers on here?

Sorry, that's one thing I have never done (and for some reason really have no desire to).

I hope someone can come along and answer your questions though!

Off the top of my head - does your niece have more confidence than you on your horse? I would suspect this is the case and your horse is feeling that and moving out more for her. Just a thought.

She doesn't have more confidence in terms of her relationship with my horse, but because she's young and hasn't had a c section she has way better core strength. My horse turns HARD but she's also very adjustable. So basically you have to give her a loose rein and let her rip, if you try to check her at all she will slow down way too much. She checks herself. But you really need to be able to sit up and stay with her when she digs down in the turn and I just feel like I get thrown around. My niece can sit it somehow. I'm trying to figure out if anybody has some good tips.

Conversely if you don't WANT to go fast she is inherently lazy so she will go really slow. She knows my balance isn't as great in those turns, she's very sensitive to your seat and balance, so I just cannot for the life of me keep her moving as fast in the turns as my niece can.

Does that make any sense?

Thank you I appreciate that. My horse is not a 'barrel horse' per se, she's a cow horse for sure. My friend has an actual professionally trained barrel horse and he's a very very different ride. He knows the pattern and turns but you can rate him with the reins. He has a lot of go and is very forward. My horse has been trained to stop, like a reining stop. Basically all you do is think 'stop' and she stops. Her horse is much less sensitive to the bit. Even in a halter, my horse has a lot of stop. Her horse has a lot of go.

But I know this about my horse and I can stay off of her face (theoretically, it's not always easy in practice at speed), it's more how do I sit those turns without getting popped out of the saddle and losing my balance which in turn makes my horse slow down. She also gets REALLY low. I think the issue is more with my core strength and balance, because between my niece and I - well we are riding the same horse here!

I don't want to put this on 'The Other Place' because the googler always tells me to stop gaming my horse that I will ruin her. But I have had her for five years and she's still lazy unless you take her to a gymkhana or to work cows. She also stops just fine after she runs her pattern and walks calmly out of the arena. So I am unconcerned about ruining her.

I found lots of work without stirrups at all paces improved my core strength, even jumping without stirrups too.

I am no pro at barrels, I've had the opportunity in the past to game and get a few lessons from a seasoned trainer on one of his crazy gaming horses.
I think the only reason I was able to stay on was because I'm used to riding english and at that point I was riding probably 4-5x a week and jumping.
Maybe that could be an option for you to try riding him english on the flats to help build up your core or if you're more comfortable western lots of no stirrup work.
What HL4L said actually makes sense, the trainer told me to drop my inside rein to help the horse do the turn and naturally I was actually probably doing that.
Anyways, what I said is pretty much just a repetition of what everyone else said

Maybe a video would help us pinpoint exactly what's going on?

OK then definitely English no stirrup work is a must!!!

Happy No Stirrup November!!! I'll be joining you with that one lol!