Riding a straight line and influencing a horse to travel straight is one of the hardest things to do! I definitely struggle with it!
The only things I can suggest and what I try and use as well, is visualization and forward, forward, forward. Look up and directly at where you want to go. Try not to do too much over correcting and make sure your horse is moving along at a good clip. :)
Opportunity is missed by most people because it is dressed in overalls and looks like work. ~Thomas Edison