Smrobs has given you the answer, I just wanted to add that I use a verbal cue to slow down. I don't know the proper "slow down" word so I just say 'Easy'. After awhile, hopefully he will slow down with just the word cue. I don't like saying whoa when I just want her to slow down.
I agree with smrobs (and others), too. We keep our mares in egg butt snaffles, and to me, having a horse that is light in the mouth (and leg) that responds quickly makes for a very easy, effortless, and pleasureable ride, and harsher bits is a last resort.
We use 'Walk' as the command if they are a little fresh and want to get to the trot, and 'Slow' when we want a very careful, slow pace like going down a steep enbankment.
If they are still a little fresh and the 'Walk' doesn't get to the pace we want, then it's, stop...back, back, back....stop...let them think a minute, and then return to walking. At least for our mares, when they have that 'I want to go' in them, I've found the 'stop/back/think' works much better than circling them.