Couple of tips:
1.) Put your horse back in a normal snaffle. If you want something with a little more bite, try the ones that have twisted wire near the center, and smooth near the ends. (When you ask lightly, it will only use the smooth part, but when you have to 'get after him', the twisted wire will come into play). His problem is self carriage; you can hold him in a collected lope or canter, but as soon as you release him he ZOOMS away. A stronger bit does not teach a horse to be lighter; it just covers up the problem because it gives you more power (and then, something for him to pull on).
2.) Always trying to 'pull' a horse into a slower motion is going to get you undesired results... like choppy strides or 4-beat gaits, or falling on the forehand. By always pulling him back, you are shutting his engine down and he responds by taking little crappy strides... lol not pretty like what we want in the show pen!
3.) Although backing and stopping is good, doing it to slow your horse down doesn't always work (as you've noticed!) Why spend all of your time in reverse, as Bob Avila says, if you're trying to fix your 'drive' gear?
4.) What enables a horse to lope slowly is his degree of collection and self carriage... NOT a trainer telling him to constantly go slow (that's how you get head bobbers!). When his weight is on his haunches and he's driving with his hocks, it lifts up his back and allows his front end to sweep out in front of him, and become lighter. I'm sure you know that, but when you ride, always think to yourself with your seat that you want his weight from the back, you want his back to round, and his shoulders to /lift up/.
5.) If his head is too low, that is not going to help his lope, nor his self-carriage... it's just going to make him heavy, which will then make his footfalls loud... and his stride fast. (hence the problem with teaching a headset too soon... been there, done that!
) His head WILL COME DOWN when he can push with his hind end. So... I know this will be hard... but in the beginning, you're going to have to encourage him to lift his shoulders and neck (still with his head on the vertical... think dressage, a little).
6.) Okay, SO! With that being said... let's see here. :)
When he's loping, take a hold of him /softly/, and pull towards your shoulders (don't lean forward!). You want his neck to curl up a little, for him to flex at the poll. Think as if you're trying to lift up his shoulders. However, as you're doing this, you need to 'push' him into your hand--if you let him slow down or 'suck back', you're just asking him to leave his hocks out behind him. It may take him a while in the beginning, but only release when his neck is flexed slightly and you can feel his back coming up and his legs coming up underneath him. A good way to tell if he's light and collected? If it feels like you could make a good sharp turn or a good downward transition without him slamming onto his forehand (a good way to check is to ask him to trot for you and see how hard his front end hits the dirt!). Once he feels in control and collected, release him for a job well done. ALWAYS remember to PUSH him into the contact, sometimes with spur to give him an incentive to stay in that frame on his own (but don't PUNISH... just make it easier for him to do the right thing). Always start out as light as you can, then increase your pressure. He will NEVER be light if you always haul back on him.
7.) A great great exercise for lifting the shoulders if done right--the counter canter. :) Canter him on the right lead, and as you go to the left, keep him in the bend of the right lead--head turned to the right, hips out to the right. This causes him to lift his leading leg higher so he can go to the left... helping him lengthen his stride. If he's never done it before, keep your circles big, and if he breaks or switches leads, just calmly show him what you need him to do again. You'll notice that when you do it correctly, in the beginning, he canters faster... because he has to take bigger strides. With more muscling, he'll begin to rock back and slow down.
8.) Slowing down will come, so don't worry so much about your speed--just work on lifting his shoulders. When he can carry his weight on his hind end, he'll slow down like a little pleasure horse. :)
Good luck! I hope this is enough... if you have any questions though, just send them by.