My horse pulls against the bit and raises her head when I ask her to slow down. Any way to fix this?

I had the same issue, I would check and make sure your horses teeth are utd you could use running atratchmemts to a breast plate to prevent excessive head tossing. patience is key (:
Half Halts my mare did the same thing and now she only does it if she has a few days/weeks off. And I did more transitions. And vertical and lateral flexion helped make her more supplier which helped with that
My horse is like that and half halts are one of the best things to do. You can't constantly pull in your horse's mouth because she'll just fight it even more. Lean back, put your weight in the saddle, do half halts and breathe deeply. By leaning back, you add weight into the saddle so the horse knows you're there. The breathing somehow makes you more relaxed and less tense on the reins. The half halts keeps your horse on a steady pace.
Half halts, and more leg. Lots of transitions. Most importantly, more leg
Maybe try asking with a softer bit so she won't resist you, maybe a French link?
Half halts and lots of work on transitions; also, try moving her from your leg and seat, rather than hands.
Pulling or bitting up is just going to cause her to brace and pull against you more - and that's a tug of war you'll never win! Do some work reestablishing your half-halts and lots of transition repetitions. Focus on really asking her with your seat and body instead of your hands as she can't brace so much against those. Asking her to bend in a smaller circle or other figures like serpentines with a soft opening rein will get her softer through her body as well.
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