Check the Auto-Slow-Down toggle in the settings panel. If you have that checked, your steed slows down when you stop pressing W.
If you have auto-run on, the horse will not slow until you take toggle auto-run off.
Also check, the acceleration rate points on the yellow side of the skill tree (shift M). Max out the yellow side to help with turning, acceleration rate, and max speed.
The other items mentioned: sling shot, stop n go, are all on Turbine's side.
Whoever says “I” creates the “you.” Such is the trap of every conscience. The “I” signifies both solitude and rejection of solitude. Words name things and then replace them. Whoever says tomorrow, denies it. Tomorrow exists only for him who does not seek it. And yesterday? Yesterday is Kolvillàg: a name to forget, a word already forgotten.
The Oath: A Novel by Elie Wiesel