I train horses, and I train people to ride horses. The best thing I can tell you, is fight your natural tendency to squeeze with your legs/knees. MOST horses nowdays are trained off the leg cues, and if you squeeze, you will probably be telling him to GO!!! That is the cue for faster. Your seat should be deep. You should be relaxed enough to feel your seat bones hitting the saddle, and as said earlier, it's always better to lean back a little than forward. Natural tendency is to lean forward & squeeze - that will tell them FASTER! As they try to get under your center of balance (you move forward, they try to get under the more forward position, lean back, generally they slow down.) One rein stop is essential. Even if they don't stop when you use it, you'll redirect their momentum, and get them spinning, and worst case - you can dismount on the spin. The point of the one rein is to halt forward motion, even if they don't stop, they're not flat out bolting. Hope it helps. BEST thing you can do is find a good instructor, and take a few lessons. Good luck!!!
Ask Often, Demand Nothing, and Reward Generously.