Your shoulders should be balanced over and centered on your pelvis for normal flat work. This requires a slight arch to your back in what I have often described as a “show the guys what you got” position, but the position must be maintained primarily by balance, so that your body can flow with the horse, not by tensing your back muscles because if you tense the upper body and back muscles, your body will not flow with the horse, and he will be carrying dead weight, and unbalanced dead weight at that.
If you feel and look “hunch backed”, you are not balanced and centered over your center of gravity and your hips, and your body is slumping forward. Your head must also be centered and balanced over your body and you need to be looking forward, while it sounds like you are tilting you head down. One of the better “centered riding” techniques has the rider imagining that there is a string attached to a skyhook at one end and the other end attached to the the center of the rider’s head, so that the rider’s head and body are hanging straight down from the sky hook. Another useful aid is to remember that riders who look down at the ground (which you must be doing) tend to end up there.