I realize that the best way to handle a horse and work with him is to be the dominant partner. If you first meet a horse and do not establish your role as leader, will that effect your future standing, or can you establish your role as leader? Once established, how often is it challenged??
Horses live in the moment. So, if you let's say, are leading your horse and you allow him to step into you on Monday then decide you had enough and on Tuesday you redirect him (say for example, every time you lead him and he steps too close you stop and back him up until he is no longer stepping into you).
You may find the horse will still step in to you a few times, but if you are consistent....every time he steps into you, you stop and back him up. Then he'll soon learn there's pressure if he steps in toward you and there's no pressure if he just leads alongside.
If you allow this unwanted habit of his stepping into you, dragging you around, etc...for a while, days, weeks, months, years....then you will come across what I call "junk" and you must go through the "junk" in order to get you and the horse through the "transition" into better habits.
So, if you let your horse drag you around on the lead for weeks, months or years, therefore establishing a bad habit (you are training him to step into you if you step away instead of telling him to back up).....you can get a horse that then "over-reacts" when you decide one day not to take it anymore.
By "over-reacts" I mean, your horse might rear up, come toward you to push you out of the way with his shoulder, try to bite you, etc.... because essentially, you told him for the past however length of time: "you're the dominant one and I'm submissive" and now suddenly you want to change the rules and so, you'll have to earn that respect. Because he'll feel the need to "put you in your place" no different than if you were any other horse. (because he only knows what instint tells him to do)
Horses, just like people, need time to get into the "new" routine. Just like we need time to memorize things for school and work, so do they...and that's where your being consistent is extremely important. Once you've established the good habits and broken the bad ones, you don't want to "lie" to him and go back to your bad habits of letting him drag you around, otherwise, you'll just keep bouncing back and forth between good days and bad days.
That said, if you're consistent now and don't get angry or over-use or under-use pressure (wail or nag) and every single time he steps in to you, you stop him and back him up. Then he'll see that you're being fair and you're leading and you are no longer submitting and he'll let you take the lead.
Horses don't want to lead humans. We're too weird compared to the simple horse.