Do you trail ride or hack in fields with this horse? I ask because by far the best movement I get out of my horse is when we're out "playing. Collection cannot happen until the horse has the proper muscling to carry himself correctly. The aforementioned playing is the best way to build him up. I know from experience that if you start going out first at the walk and then working up to trotting and cantering, assuming your position and rein contact is correct and supple, each ride will produce more fluid collected gaits. The added bonus to being out of the ring is, it automatically gives you the forward component necessary for collection. If you don't have forward, there's nothing to collect. There's usually plenty of engine there and all you have to do is keep yourself balanced properly over you horse and your rein length correct and supple, and the collection will come by itself as your horse finds his balance with each subsequent trail ride. I'd start there because you need to feel what it really feels like to you don't accidentally spend years in vain in the confines of a ring for collection that will never come. Once you feel your horse really working from behind in a big powerful trot, you'll be able to start reproducing it in a ring.
You just have to see your distance...you don't have to like it.