#9 Not researching and targeting the right agent
Confidence is not my problem, but not matching the right agent with the material definitely is.
One of the mistakes that writers do is using the "shotgun" approach while querying without doing the needed research on the agents. The shotgun approach only works if you're shotgunning the right agents. Otherwise, you're just shotgunning for a whole lot of rejections. It's perfectly fine if you're trying to wallpaper your bathroom.
It's also a mistake to query your top, dream agents first. Why? Because most people simply aren't ready yet when they first query. The query letter isn't ready. The pitch isn't perfect. The sample chapters aren't polished. We're not sure what exactly we're selling, yet, and how. We need practice runs, just as an Olympian wouldn't make the Olympics his or her first competition, or an astronaut wouldn't fly the Space Shuttle without tons of simulations and dry runs first. So why waste your chances with your top agents with a lousy query? Sharpen your skills first. Practice. Then when you are ready, prepared and confident, go for the kill.
However, it's still difficult to find out exactly what an agent represents unless a) you write a specific genre such as romance or fantasy, b) you're familiar with the books and authors the agent represents (and providing you can get their client lists), and c) you know they're still representing that genre. Unfortunately, as I found out the hard way, many agents list "literary/mainstream fiction" as the genre they represent but, in reality, it's not. If the agent says "literary fiction" and you keep seeing romance and fantasy in their client list, it should tell you something. So yes, do your research. Make sure you're querying the right agents.