It is easy to keep clean, very comfortable, not expensive and will not slip. Just because you think you are relaxed doesn’t mean that she does. Not having anyone who could help you is difficult. Most of us can only make suggestions based on what has worked for us and our horses, since we don’t know you and yours.
Can you work her in hand? Long lines, using side reins (unless she keeps her head where it belongs, which mine do not)? Or, perhaps, try a bitless bridle when you ride? Maybe you could find an endurance saddle that would fit you both? Once you know what size and style is best for you, you could check ebay for a used one, which would cut the cost considerably. If there is no saddle fitter near you, you could consult your vet.
It is never the horse's fault