Well,
as I am at that point about once every two month, here is some advice:
Challenging the characters by encounters is no longer fun because you know what your players are able to do and can built the encounters just right? They are neither over nor under challenged? Try to add some randomness and stick to it. Use the Crit Cards from Paizo or those Swashbuckling Cards and use Random encounters instead of prept encounters. The table gave you 4 Frost Giants? In the middle of the Desert? Against 5 low level heroes? All dwarves who think giants are their racial enemy? ... I guess thats a challenging situation. Try to work with it. You will be suprised how much a little chalenge can add... (Of course, maybe your encounter tables should not include Frost Giants in the Desert... btw. Mother of All Encounter Tables by Necromancer Games is great for this... I had to fit a Behir and a bunch of Dire! Goats into a Mountain Pass... it was a great encoutner
)
You know everything that might happen during the session? Because you planed for everything last monday and your players just are not the kind of folks to ruin your day? Hand out index cards and let every player write down something he would like his character to do today. This can be as simple as "Kronk wants to smash through enemies, finally putting his great cleave feat to good use, which he was not able to use since the start of the campaign!" to "Kiara wants to meet her brother Cedric, whom she last met 10 years ago, by that time he left home to become an andventurer". Be sure to mention to your players you might not be able to introduce even half of what they give you, but do it every session. Allways try to add at least one thing into the running session. Bonus: You will have a pile of ideas to work from for session prep.
You know what will happen in your campaign 10 month from know? You allready know the end, to you it feels like watching the whole "Firefly" Season for the 20th time draged about the course of a year? Simple solution first: Do not plan to far ahead. You will allways subconsciously railroad your players to stay with your plot. Do not create a plot. Just create events and wait for your players to react to those events. As you allready have a plot, try really hard not to force your players to follow it. Before every session starts, ask yourself, is there a point in todays session where my players have a real choice or am I just taking them for a ride in my big story?
Last time this happend to me, my players were about to attack a Drow Outpost. I thought they would attack, kill the drow, take the info and leave. This is what happend: The first guardian, a Gauth Beholder and a bunch of Minotaur Zombies overwhelmed the characters. Instead of killing them, the Drow captured the characters (Allways take every chance to change your campaign, worry about consequences later). They were able to flee when the Drow where attacked by an army of Kuo-Toa, 3 sessions of trying to find equipment in the Underdark followed (These were entirely player driven) and when they finally went back to the outpost, it was in the hands of the Kuo-Toa...
In essence, try to figure out ways to challenge yourself during the game. The problem with being a DM is, if you have nice group of players who would not ruin your plans or if you are very good at creating fail proof plans, you can guarantee yourself a good session by preparing enough. You can prepare every encounter and hone it to the point where it will just take the right amount of resources, you can lay out the pacing of the evening before the players even arive at the table, you can plan the entire story for years to come, so you will allways give away the right amount of information, your players will never be left in the dark nor will they just skip parts of your campaign because there is nothing more for them to learn... in a way, you invest a lot of time to bore yourself. Worse yet, as you get better at doing all that, it will not even cost you a whole lot of time...