This is me completely. Only thing is, I almost always successfully make it into a song. Don't know why, but I do have an excellent ear.
Now what I usually do is this:
- I get the song in my head, but don't rush to an instrument right away. I keep going, making up other sections of the song and just imagine in the most natural way what the lead melodies are like. At this point, arrangements and instrumentation are bound to show up. Keep them in mind but don't focus on it.
- pick up the phone and record just my humming. I don't worry too much on harmonies, but if they're essential to the melody I usually just hum the harmonies as well.
- Pick up the guitar and just find the main bass notes of each chord. Cause this is where the idea might fail on you and it's suddenly gone. If you add actual chords, your mind has a tendency to follow the guitar instead of the melody in your head.
- Carefully record one section of the idea at a time. Keep it as simple as possible. Make sure your maintain the original idea that was in your head.
Most often, the initial idea you get in your head is all about melody and rhythm. That's what makes up the core of the idea and that's why you feel it's a strong idea. Make sure you capture that idea! It will drive the rest of the creation process.
Also, songwriting is all about inspiration and the drive you have at the time. If you loose the drive, you loose the idea. But if it's a really good idea, it will stick. And you'll get the same drive the next time you go work on it.
Some people find that getting some space from the song is important. I agree and disagree. If I'm working on a song, I keep working on it until it's done. I'm obsessed with it at this point :p
But it needs to have a certain shape before I'm done with the rough sketch. And then I introduce it to the band or any other collaboration I have on writing. And then we figure out the details together.