HOLDING her will only make things worse.
I think if you can stop/slow her with your seat, that's great, but chances are, she will not listen to seat alone, but do start there.
So, core gets firm, drop your elbows down, exhale through your mouth and make an "Haaah" sound, it can feel a bit like coughing, only slowed down.
That's the first cue to slow, which she will probably run right through. You apply rein, now.
Make sure you have one rein just an inch or so shorter than the other, it's called "offset", and this helps to make it less easy for her to lean on the bit.
Start taking up the reins and where you get to the place where there's a fair amount of contact on it, you stop taking up the rein and you make your hand very , very firm, as it it is made of concrete. This will feel to the hrose as if you warned her, little by little, and then she ran into a brick wall. Do NOT jerk! Slow uptake of pressure, then just STOP your hands.
It helps to think of them as becoming anchored to a spot on the ground. In fact, you can, as you are beginnning your halt, pick a spot aht is a few steps ahead and say to yourself, "THATS our stopping spot." and when your hands pass over that spot, your hand STOP, as if they hit a wall.
Your horse will feel this firmness and she will lean, or she will bounce off. Your job is to wait unchanged until you feel her "bounce off", even if it's very little. You will feel her rock her body backwards a bit, and lift her head, so she's not plowing to a stop with her whole head and shoulders leaning on that bit.
When she does, make your hands soft, and give her a couple of inches of rein. Tell her she's a good girl. Let her stand for a sec, if she will, then ask her to move on, but make this only through your seat. Just "allow" her to move on.
She'll likely start speeding up again, and you do the whole thing again. Pretty soon, when she speeds up, you just firm up your body and she'll stop from that. But it's very important to AlWAYS start your halt with your body, THEN your hands.