I think the root of the problem is that no one ever put you on a longe line and took the time to properly position you. Your body is like a tower of building blocks stacked one on top of another. If even one of those blocks is our of line then the tower is weak and likely to collapse. If all the blocks are lined up then you have a strong structure. The blocks for work on the flat should be ear, shoulder, elbow, hip, heel. Your head is up , your shoulders are back so your chest is open, your arm hangs Straight down, the elbow is bent at close to a 90 degree angle and is a fluid Hinge! joint, the elbow sits just above your hip which is open such that your legs Hangs down and around the horse. The irons are just a place to take the weight of your leg, the weight drops in a natural fashion right down through the heel. If I came and took the iron away your leg should feel like a over cooked chicken leg, like I could rip it right off at the hip joint. It is that relaxed and giving when not being used to give a command. That way it can drape around the barrel of the horse. Pull your foot out of the iron and ride for a few rounds at a walk with a `cooked chicken leg`. Then take up the iron again. You should be able to feel the weight fall into your heel a bit more. To open your front do a Military shrug, that will set your shoulders where they need to be, remember to breath deep right from the diaphram ( in through the nose) and to shake your head NO and nod your nose YES to keep from stiffening your neck.