Hello MT!

Lately I've been spending a lot of time trying to compose and refine songs that mean something to me. I like to set goals every now and then but with music, I find it hard to come up with achievable things that I should work on in order to improve my music.

It's easy to say "ok I want to understand how this progression works" or "work on this technique", but how can you judge your own originality and creative quality in order to improve it?
When you're satisfied with something you have composed play it to other people and ask how they like it
I have to disagree respectfully with HaistaPaska, of course playing it for people will get one kind of feedback, but it isn't the self-evaluative kind.

You have to give yourself some distance from the piece, in my opinion, then come back to it and examine it from as neutral of a perspective as you can. When I write, I often will complete a section, put it down, work on something else, then when my mind is focused on that one (usually after a few weeks), I take a moment to look over the old piece and see what I feel about it then. I often find that sections I was very attached to when I was writing it don't feel as important when I look back over it, and some things I wasn't sure about actually work really well. Then I make notes about it, finish doing the other piece, then go back and revise the old one.

On the other hand, you might not want to revise, it might be a better use of time for you to just make notes, and move on, incorporating what you learned from the distanced evaluation into works you will write later.

Most importantly, never be afraid to throw out anything, very few great masters wrote masterpieces in one draft. You look at Beethoven's sketchbook, and its incredible the amount of material he will write in a draft, then just completely discard abot 80% of what he wrote, then he'll rewrite it and discard 70% of the next draft, on the next one 60%, etc. Don't get so attached to a piece (especially one you are just writing for practice) that you are afraid of throwing out what isn't working.
Generally if you write more you'll write better stuff as you go along. But I'm supporting "listen to the audience". It's not like people create crap music/movies/art/books on purpose, they're just too "close" to the work to make an evaluative judgement.
And no, Guitar Hero will not help. Even on expert. Really.