Ironically, to get your horse on the bit, you need to forget about everything from the neck forward in the initial steps. Your basic young horse who can W/T/C is operating the way he was built. The front legs carry the weight, the hind legs propel the horse. When you ask for collection, you're changing the way the horse uses his body and asking him to use his hind legs to bear weight as well as propel the horse. To do this, you have to train him with exercises that result in weight being transferred from his front legs to his rear legs. The most basic way to start doing this is to use transitions both between gaits and within gaits. Walk-trot-walk tends to be the most effective for starting out. The key is that you as the rider, needs to be capable of understanding how the horse if moving beneath you. With each transition up and down, you should be able to feel your horse working more from behind. If you just do what I call "empty transitions" very little will be accomplished.
I'm already getting wordy, so I'll stop there for now. Beyond basic transitions, there are many more exercises involving circles, serpentines, and lateral work that will get more weight transferred to your horse's hind end, but I'll see if all the crap I wrote in the first paragraph helps first.