Re: The first note always sounds "right"
I think I know what you mean (although not exactly). This is that game we play with the analytic side of our brain v.s the intuitive side.
In my experience the best melodies I write just come out of nowhere. But some times you also gotta tinker with a melody to get it sounding 'right'. It's pretty normal.
If you get stuck best thing to do is give your brain a rest. Get a drink or something. Try not to think too much about what you're writing but just have it floating in the back of your mind. Hopefully, when get back at it you'll A) Hear it differently and know what to change (or not) B) Realize it's better to start over, there's plenty more ideas where that came from