My barrel horse over-flexes when he turns a barrel, lopes in circles, and even when I ask him for figure eights at a trot or lope he is too flexed. He has been doing this for a while and it is starting to affect his running style. I would like expert advise for this problem!
P.S. I am a HUGE FAN! My favorite of your horses was and still is Seven.
First off thank you for remembering Seven. He was my favorite horse too! Regarding your problem. The good news is that you can almost immediately resolve your problem by using the outside rein to help balance and stabilize your horse's body position. Remember that 50% of your control comes from the inside rein and 50% comes from application of the outside rein! The inside rein is used to cue for direction, the outside rein helps to stabilize your horse's head position and balance your horse's shoulder position.
It sounds to me that you are throwing away the outside rein and just trying to control your horse with the inside rein.Try using both hands on both reins to stabilize and position your horse's head and shoulders. At the same time use your legs to squeeze your horse forward.
You might try to use a bit that is stiffer than perhaps a ring snaffle or gag type bit. Remember the stiffer the bit the straighter the horse and the more front to back control you will have. The more moving pieces a bit has, the more lateral control you receive, conversely often too much bend. Imagine where you would like your horse to carry his head, then use both reins to place it into that position.
An additional suggestion: Elementary Dressage lessons would help you understand the mechanics of using your seat, legs and hands to help balance your horses body position. He will be a happier and more confident horse too.