Based on your description, I see a couple of areas you can improve relatively quickly without having to think too hard about it.
Keep your communication efficient.
Start by reading up on the basic writing concept known as 'Chekhov's Gun'. The general idea is that when engaging in dramatic narrative, you should only include details that are actually relevant.
This obviously can get very boring very quick, but it's important to remember as a counterbalance to the tendency of a lot of newer GMs to engage in long prose-like descriptions of everything. In a well written drama, you only include irrelevant details when they actually contribute to the mood of the story in some way (for example, in a Western nobody cares what the bandit who just rode into town had for breakfast, but the time of day and the weather help set the mood for the ensuing gunfight with the town sheriff).
The same principle can and should be applied when running a game. If a given event isn't seemingly significant to the players and further detail won't be relevant to the game in some way, just keep your descriptions terse and to the point. 'You miss' is perfectly fine if the party are just in a low-stakes fight with a bunch of mooks, but if that shot hitting would have clearly changed the course of the battle, then you might want to get into detail about how and why they missed.
Note, however, that this only applies for fine-detail. If you're looking at actual large scale plot, absolutely apply Chekhov's Gun to the greatest extent possible without violating the Three Clue Rule (which sounds like the opposite of Chekhov's Gun at first, but actually advocates the same principle in a slightly different way and should honestly be required reading for any new GM).
Keeping your communication as the GM efficient like this is honestly one of the single most important skills as a GM. It helps your players keep track of what's actually important, it helps you save creative energy for when you truly need it, and it helps keep the game moving because you'll end up talking less.
Consider taking breaks during your sessions.
Get up and walk around, get a snack or a cup of your beverage of choice (well, maybe not a beer or a glass of wine, having an intoxicated GM can be entertaining at times but is usually just a pain in the arse). Anything to just stop for a bit so everyone has a chance to be refreshed. Most likely, your players will be more than happy to take a short break halfway through a session (or if it's a three hour session possibly two breaks). Bonus points if you can time things so that the breaks happen when there would be a commercial break if the campaign were a TV show, it's a great way to help manage the flow of the narrative (especially in drama or horror heavy campaigns).
Alternatively, I've found that starting out with everybody having a meal as they start playing can help in a similar way, as it tends to slow things down a bit at the beginning of the session so people don't get burned out as fast over the course of the whole session (also, the slight increase in blood sugar from having just had a meal helps keep people energized).
Don't forget about your own planning time.
You mention you're doing 3 hour sessions once a week normally. Think for a moment about how much time you as the GM actually have to spend over the course of a week. Chances are if you're like most GMs it's at least as long outside of the session itself, which effectively means you personally are effectively doing six hour sessions once a week, not three hour sessions.
This is one area which a lot of newer GMs don't think about much, but it's one of the biggest contributing factors to burnout for a GM. There are two approaches you can take to dealing with it, either shorten your sessions (and thus reduce your prep time because you don't have to fill as much time with content), or learn to reduce your prep-work and ad-lib more (this gets easier as you gain more experience as a GM). The key here is to reduce the amount of work you as the GM have to do without reducing the quality of the campaign from the perspective of the players.
Always remember that you are your own worst critic.
This is just an important life skill, but applies here especially based on your description of your issue. A relatively basic part of human psychology is that most people, when they have some standard to compare against, will almost always compare themselves unfavorably against that standard unless morality is directly involved (in which case most people will tend to compare themselves favorably against the standard even if they fall far short of it).
You've got a group you're GM for. They have said they are fine with how things are right now, ergo, you are an acceptable GM for that group independent of how good you are compared to other GMs.
Wanting to improve is a good thing, but you shouldn't beat yourself up over it, that energy is much better spent actually trying to improve.
