you talk alot about collection. I am not sure I would say she is "collected", but I think you mean she is flexed to the bit and soft on it, right?
If you are interested in learning some cool stuff from a book, consider buying and reading "True Horsemanship Through Feel" by Bill Dorrance and Leslie Desmond. YOu will find this very hard to digest at first, but keep it and reread it and rearead it and . . . each time it will make more sense to you. I think you have a light and sensitive hand and if you can develop this "feel" it will make your horsemanship grow hugely, and deeply.