1. EQ // get your sound clean
2. Compressor / Limiter // control dynamic of your sound
3. Multi-band Compressor // do 1, 2 at the same time which can make greater effect.
4. Reverb and Delay // add depth, sense of space
5. Stereo Imaging // mono compatability, wider sound.

These are almost everything.

You need to work making a good, clean mix. It doesn't need to be very big and loud. I think this is the most difficult part.

I had same problem. I could write some 8-bar loops though.
I would write a loop, try to expand it to a full track, and throw it away.

I found it's mainly because I skipped "songwriting" process.
It's very important to sketch how you'd like to expand your idea.
I still need to work on my songwriting skills, but everything went a lot smoother after I started thinking about it.

I don't think any music theory or technical skill is very important to finish something.
So just try to make any thing, but try to sketch your idea as much as you can.