You allow too much freedom to players in the creation of their characters. You tend to assume they should be able to draw freely from the whole context and that it's the GM role to arrange for the party to make sense and have some reason to exist as such. You really should learn to say "no" to some bogus concepts, the ones likely to actually impede the game later.

You know the rules. You know the rules you want in and you know the rules you want out. You also handle rather well how to gradually introduce the rules to new players. You've never found yourself desperately browsing through your books in search of a specific bonus value and when you don't remember something, you can often guess pretty accurately what it should be. It's one area where playing a game like GURPS helps a lot: the rules make sense, and this in turn makes it easy to make up some rule and see whether it remains consistent with the rest.

You sometimes prepare what you like instead of what's actually needed. Too often you realise that you have prepared a carefully worded letter, or an extremely detailed NPC, where some key element you have left to improvisation. You're not bad at improvising, but you're not great either and it's likely players can feel the difference between something well prepared and something made up on the spot, like crossing an invisible veil. Of course, it's even more obvious and hard to compensate for when you run solos. Since there's only one player, there's no debate about whether a plan is sound or if a way is safer than another. The decision is made real quick, and there's no time to jot down a few notes, or rearrange the scenario.

You read a lot of material, from many games and from many sources. This helps to avoid getting trapped into the same old design reflexes and the same old campaigns over and over again. In other words, by listening what others have to say, you remain alert and open to new ideas. Also, you almost always manage to get something even from articles or books you don't appreciate very much.

You read a lot, but you don't play a lot. A busy life, a tight schedule, the lack of players: you have all the excuses in the worlds, but in the end, you just don't play as often as you would like to. You feel this lack of practice reflects on your skills as a GM. Your descriptions are good enough but could be better; some improvised NPCs have a bit too much in common with each other; some unexpected but excellent ideas raised by the players don't get the attention and the reward they deserve… Even though the players don't come to you complaining about this or that or anything, you really feel there is room for improvement here. Also, you always play as the GM, and it's been years since you last played as an actual, regular player. This also has an impact on how much you feel you can connect with the players.

You always try to have something for everyone during a session. This means that you try hard to make sure there always is an opportunity for each PC to shine: the retired Lieutenant gets a chance to tap his network in the army to work around some obstacle; a courtesan will have someone interesting to seduce and the naturalist will reveal the subterfuge of a crime, where the poison is not from some common snake but from a rare cross-breed between a viper and a cobra, a species a specimen of has been found in the house of this particular NPC… This also means that you try to give something to each player, whether they are butt-kickers, casual gamers, method actors, power gamers, specialists, storytellers or tacticians (to follow the seven types of players identified by Robin Law). Perhaps you used to do that intuitively before, but now that you've read Robin's Laws of good Game Mastering, you finally have an actual method you can rely on to proof-check your scenarios.