You'll get there. It's tough, and it comes with time and practice, and you'll probably have to force it at first. If you're like me, you'll develop a style that reflects a bit of your engineering background.
I started out having a VERY difficult time making anything with the "grunge" look - if left to my own devices, everything will be clean, geometric, symmetric, perfectly measured and aligned (I get out the calculator to figure out exactly how many pixels each frame will be). But I really LIKE the grunge aesthetic, so I started working with other people's pre-made textures and patterns and stuff, and now I can incorporate those styles into my own designs reasonably well.
Look at other designs that you like - really study them, look at all the little details. the gradients, the drop shadows, the frames and borders, the overlaid textures, etc. All those little details are what makes the design look "finished" - they're all little individual components all stacked together to make the final design. You can accomplish a LOT of fancy-looking stuff simply by applying layer styles to a very simple font or line pattern.
As stated above, I use a lot of pre-made clip art, textures, and patterns. Most of it is free downloads, but I've bought a few things off iStock and shutterstock. Deviantart is a great resource too, as mentioned above.
"Start to learn Illustrator" is on my 2013 to-do list.
Helpful hint: As you're doing your projects, start building up an organized library of design components, clipart, textures, etc.
Helpful hint 2: Google for "free vector [SUBject]" and you'll usually get some good hits. A vector is an image that can be infinitely scaled up, so you don't get pixelated results when you blow up a small component to take up the whole page. (warning, some of the "free" results will actually be "royaly-free" which have a one-time payment - e.g. shutterstock, iStock)
Helpful hint 3: If you like the "retro" aesthetic, check out http://tackorama.net/
for fonts, patterns, etc.
