I have a western riding question?

Ok, so I have been riding western saddle for about a year now and I have a few questions. I am in training for barrel racing. I seem to be getting the part of heels down, sit back in the saddle, rock the reins with the horses head when at a lope, move with the horse. But I can’t seem to get the part of relaxing my legs better and doing what I would call slouching, or as my trainer puts it, getting punched in the gut. Is there any exercises I can do that will help that? (relaxing legs and slouching) I am a tall person, so it is hard for me to slouch because of my good posture. Also what I am loping and I make a turn, I can’t seem to turn my upper body on the outside, so when I go around a turn my trainer says to lean the opposite way. When I am turning I go way forward and end up almost falling off. I lose reins, my body falls way forward. I haven’t fallen off yet when doing that stuff, but I am counting down the days till. Is there anyway I can better that? Thanks!