Ditto all the above, its sound advise from very experienced people, please don't take offense to suggestions as all any of us can do is respond based on the information you have given and what we know from previous posts, although you do well on other horses and RS horses some horses have a slightly more difficult streak or are less well broke and you do need to have another level of skill to work with these and that is no one being rude, but you may find lessons on this particular horse really do help you develop the skills needed to work with a horse that isn't straight forward.
In the mean time there are a couple of things you can try which I use on my horses that do both western and English.
Firstly if the horse was primarily trained western you need to check your position, western horses are generally trained to be more sensitive to weight and pressure, so check you are sat deep and soft with your shoulders back and your legs at most very softly against his sides. The last bit there is the 1 biggest thing I note they struggle with when switching a horse from western to English (that and accepting a constant contact) western lays your leg in a very light contact, English, by the nature of the stirrup length (especially if you are riding jumper length) puts your leg in a firmer contact with their sides without even trying, you may need to drop to a dressage length and gradually bring it up, that normally helps.
Second as EML suggests half halts or rating him periodically should help, a constant pull will just give him something to lean against so should be avoided.
Third I agree with Skib, doing just a few strides at a time with literally 100's of transitions will help getting him not always thinking about rushing forwards all the time and get his butt under him ready to stop which will naturally slow him down, additionally lots of shapes and changes of direction (combined with the transitions) will really get him listening to what you are going to ask next, just running round the rail really encourages them to rush and motorbike rather than get their butt under them and control the power.
And you can try humming or singing through sitting trot and the transition into canter and throughout the canter, this helps relax your diaphragm & lower body which helps deepen your seat and western horses often respond well to this.