My horse is a western pleasure horse...sometimes he goes the speed he is suposed to (slow) but sometimes he goes way faster than he needs too...how can I get him to always do the right speed...what makes him go too fast???
How long are you asking him to canter slowly for? Maybe he is getting bored and speeds up? Work on transitions or patterns to break it up andsee how he goes?
There are things you could be doing to make him speed up like leaning forward. I ahve the bad habit of leaning forward in corners (I ahve no clue why) and then I get frustrated when my mare speeds up. Are you keeping a nice even seat? Are you putting your legs on him unknowingly? Maybe you relax your reins and he takes that as a cue?
If you supply more information about when he speeds up (like in the beginning when you first ask or after a few laps etc..) and how easy it is to bring him back you will get better answeres. Good Luck!
If your horse is in shape and physically can lope slowly, make sure you aren't changing your ques or riding position to envoke a change in speed. If you are staying relaxed and sitting deep in the saddle, I would suggest stopping your horse when he speeds up and immediately asking for a couple of steps backwards. Then ask for the lope again and if you get the speed you want, leave your horse alone and let him lope. If he picks up speed again, stop and back. You want to make the right thing easy and the wrong thing hard.
If a horse isn't doing what you ask him to do, then you aren't asking him correctly. I would suggest getting a trainer to watch what you are doing and make the corrections.
If your horse has the training to do what you want - then it's up to you to control him to do it. Telling you how is next to impossible unless we can see what it is that you are doing (or not doing).
90% of the time (maybe 100%) it is the rider and not the horse - assuming the horse knows how to do what you ask. Asking someone to tell you how indicates that you need a trainer.
As far as the statement about "horses having their own minds and not expecting too much" - then there is no point in training. If you don't expect too much, that is what you will get and you will eventually end up with a real problem horse.
If your horse can lope slowly, which you said he can, then you need to give the proper cues to get that WP lope when you want it, not when he decides to give it to you. Basically you are at the horse's mercy right now when it comes to the lope, don't be embarrassed, we've all been there to some degree. You need an instructor to teach you the proper cues, as this goes way beyond the realm of this forum. Once you're working with an instructor, you will have so many "AH HAH" moments, you'll wonder why you didn't do this sooner, I promise you.