Because my horse is massive, respect was key for me. He can be very sensitive (fearful) so I quickly learned that I couldn't simply correct the bad behavior but go back to ground work to try and prevent it (be the alpha in the relationship so to speak).
The biggest thing I found to help was lead work. For example when he came into my space or didn't stop immediately he was made to back up then stand, and wait for my next instruction. It sounds simple, and it is. And you've probably already done this. I just found that it helped his general attitude. Now he'll match my pace, stops on a dime and doesn't try and run his bulk into me and others. Nor does he try to rub, bite or knock me with his head.
When he's really naughty on rare occasion now, he gets a quick check then I try to diffuse the situation and quickly turn it into a positive by a minute or so of lead work then try working through the problem situation again (trailering). Also I do excersises to bring his head down (calming technique to decrease the adrenaline and increase endorphins).
Small things, but they help me. Patience and repetition is key for me.
This is excellent advice. In the winter, my barn nearly snows in, and the horses don't get the exercise, physical or mental, that they should when the weather's bad (no shelter in pasture, so it's stable or skinny with pneumonia in a blizzard). I will take 15 minutes to half an hour a day and just practice walking, stopping, and turning calmly and with respect. It honestly does wonders for their attitudes in the barn and outside once the weather does cooperate.
As far as the line between respect and fear goes, you're horse will tell you if he/she's afraid. Usually, truly disrespectful horses need a little "fear", fear as in a knowledge and understanding that if he doesn't do as you ask, there will be consequences. They key is knowing what a disrespectful horse looks like, as opposed to a horse that is acting out of fear or pain. Then there is still the line between disrespectful and outright aggressive, which, IMO, is a problem that needs a pro's intervention.
Personally, I'm big on discipline. These animals outweigh me by 10 times, and I can't afford anarchy
. Discipline does not mean a "nasty" handler who reacts out of anger, or horses who live in fear. Discipline means that the horse knows what he absolutely cannot do, and he knows that the handler will be consistent and not confuse him on those points.