HI,
Another thing you could try while riding are draw reins in addition to your regular reins. Run the draw reins (you can use a set of western split reins if you don't have draw reins) from your hands through the bit going in towards your horses face then attach them to your girth just under your saddle on either side of your horse like side reins. Hold the draw reins between your ring and little finger (where you would hold your regular reins), and hold your regular reins outside your little finger. This way you can regulate the length of your draw reins without compromising control. Start out with them loose and then gradually tighten as needed.
Also, try asking for the transition in a large circle instead of a straight line.
Good luck, let me know how it goes!