Horse breaking is an art, a slow dance between horse and rider than can end in success....or failure.
The difficulty depends on several factors.
Number one being YOUR experience as a horse person. A horse person well qualified to break a horse *in my opinion* is one that has handled and ridden many different types of horses. From the dead broke lesson horse to the spooky, green broke colt. You do need a certain set of skills to be able to handle a possibly terrorfied, high dangerous colt or filly whom has the saddle on his or her back for the first time in their life.
Number two is the horse itself. What is it's personality like? How was it handled and brought up as a baby? Is it dominate or submissive? Flighty or calm?
For example, you have two different three-year colts ready for saddle training. Colt #1 was handled frequently, halters, and leads. He is a generally confident colt, but bathing and being tied scares him, he is a bit spooky. He is a submissive horse.
Colt #2 was handled as a baby and as a yearling, but has since then not been handled. He hates being haltered, is a pain to lead, but stands tied alright and does somewhat okay with being bathed. He is calm. He is a dominate horse.
The two colts do not seem much different than each other, when in reality they are worlds apart. You may be able to break Colt #2 easier, but could have serious issues with showing him that YOUR the boss, not him. He could be dangerous if not trained properly, and has a chance of becoming a very disrespectful kicker. You need a high level of confidence, determination, and skill to turn him into a well-rounded mount.
Colt #1 may take some more time to teach, but he could be also be dangerous if not properly taught to be confident. If he is aloud to much confidence, he could turn out like #2. Not enough, and he could become frightened of the littlest things. Handled or trained to roughly, and he could become terrorfied of people. He needs a confident, but sensitive and intelligent trainer to bring out his fullest potential.
A very important skill in horse training is a knowledge of equipment and horse behavior. If your colt kicks, you need to be able to identify the cause and correct it before it becomes a habit. If your colt doesn't lead correctly, you need to know what kind of halter and what course of action should be taken in order to teach him to do so.
An incompetent trainer can lead to a dead horse. It's vital that you have a level head and aren't swayed by stupid training suggestions such as 'you gotta tie 'em up real tight like in the barn then put tarps and fans all over to desensitize them!' or 'if you leave the lead on in the pasture, they'll get used to it'
A horse should never, ever, be started under saddle before at LEAST 2 1/2 years and ideally 3-4 years. NEVER put anything heavy on a yearling and NEVER EVER mount any horse under 2 1/2 years old. You can seriously injure or ruin the horse.
There are many things you can do before a horse is ready for the saddle. Work with him with scary things such as cars, tarps, cows, birds, dogs, water, etc. Work on haltering, leading, picking up their feet, standing tied and bathing.
The most important tip of all is of course to be patient, calm, and clear minded. Frustration, anger, and irritation are things you cannot avoid but don't ever let them overtake your judgement. Quite, consistent sessions will get the point across much better than loud, irregular ones.
I'm in the process of breaking out my own colt. Good luck to you! Horse training isn't an easy job, but it sure as heck is a rewarding one!
Wait! I'll fix it....