Self Carriage is the Art of Letting Go
Self-carriage is putting your horse into the rhythm, balance and shape that you want and then letting go to allow her to maintain that rhythm, balance and shape on her own. Letting go is an art. When we let go of an aid, we don't want to let go so much as to totally drop all communication with our horse. But we do have to let go if we want self-carriage to be possible.
At our barn, we often talk about the aids as being a box around the horse. Instead of thinking of holding the horse on your aids, think of holding your aids around the horse. To let go without dropping your horse, think about your box of aids floating in the air around your horse. Your body and aids maintain the shape, the balance, and the rhythm, while allowing your horse the opportunity to maintain these qualities on their own as well. Yet your aids are still close by to help your horse when she falters (and she will!) So if your horse loses engagement or slows down, your legs catch her. If your horse's body loses bend, the shape of bend in your body reinforces it in her body. If your horse's shoulders fall down or in or out, your rein(s) are there to correct her balance and straightness. And after use, your aids go back to hovering.