Try to dretch for a week or two, without playing Human at all.
Learn to find that wallwalk key (default "C"), and to toggle it off when necessary.
Stay above the Humans, and try to touch them at head height by wallwalking at that height when they are fighting your teammates and can't keep an eye on you.
Stay together with other Aliens. A swarm of dretches swirling around some hapless Humans is a lovely sight to behold. Find somebody good and follow them or spectate them and learn their tricks.
You can mess with mouse sensitivity and wallwalk pitching, but after a couple weeks of solid dretching, you'll at least know what you're doing wrong. And you'll have a better appreciation for the more advanced Alien classes (they're not really any easier; dretching is entry level Alien gameplay).
If you can get above the Humans and drop on their unhelmeted heads, it's a great feeling to know all you have to do is touch them one more time to kill them (if you can do it before they hit the medkit button).
The ground headbite is a great skill to learn.
After a couple of weeks as a dretch, play Human again. You'll get more out of it now, and you'll be a better dretch when you go back to Alien.
Just my humble opinion, I'm pretty new (maybe 3 1/2 months or so).
Also, another person recently got
this advice.
P.S. I thought something similar to what you asked for when I started, like "Why can't I hit anything?", but with only 25 health, little mistakes are fatal. Remember, if you can't dazzle them with brilliance, baffle 'em with bullshit. Keep 'em guessing, and keep moving. Never run straight at a Human, especially on the ground. That's like an ant running at a guy with a gun. He's gonna go sidestep, turn, sidestep, turn, and shoot your bug self. Confuse and confound 'em. Find the weak Human, and separate him from the pack. Cheers!
Dretchkill, FTW! (Also fun as hell. I dretched a battlesuit for the win on ATCSZalpha, in the vents, when everbody else was evo'd up to goon and rant, and couldn't get at him. As a matter of fact, play on that map, and stay in the other team's vents. There are some spots where you can drop onto their head from just above, and if they are already less than 96 health, they croak. Mwahaha!!!)