I ride a horse exactlly like that!
She's a white pony named Porsha (do you go to a stables named PV Stables because it's possible we ride the same horse!
I used to have the same problems
Porsha always wants to catch up with the other horses and sometimes refuses to slow or stop
Here's what I did:
When she wants to pass other horses just go infront of them, she probably likes a good challenge and likes to be ahead most of the time.
When she refuses to slow remember to sit back on your pockets (horses speed up when you lean forward), pull your reins back and say woah. This should do the trick
For the big spook it's possible something scared her or she got stung by a bee
Or maybe you where doing something that upset her
One time I was riding a pony named Cookie (and she is REALLY hard to get going!!) and we where cantering to a jump. I hit her with the crop to get her going a little faster so we could make it over the jump. She does NOT like the crop! She sped up really fast and starting bucking and I think she might've reared. I lost my stirrups and reins but I hung on with my thighs. I had no controll over her. She Galloped right over a jump. Finally, I got a hold of the reins and stopped her. And now I know NEVER to hit her with a crop again!
So maybe you did something that upset her. But anyways hope this all helps!