Song Writing Hurdles

I have a two week deadline to complete an entire song. Music, lyrics, the works. Not to be misleading with the use of the word "deadline", which implies ramifications if I don't complete it. There are no ramifications beyond personal displeasure and potentially disappointing my vocal coach, being the person who tasked me with this undertaking. That being said, consequences or no, I'd like to complete it. It may be terrible and it may be embarrassing, but it will be finished, and that is an achievement of sorts. By being complete I'll be able to leave it behind and try again, hopefully making my next attempt slightly less horrifying. Then I'll repeat the process a few hundred times until I hopefully have something mildly passing. Or more ideally I'll discover I'm a natural prodigy and write a masterpiece, but I'm not going to hold my breath for that one.

Having already squandered half of my two weeks, I recently realized why I'm struggling to make more progress. I'm spending a lot of time on minute details that probably don't matter. Or don't matter at this stage in the process, at least.
Last night I spent an agonizing amount of time on 4 measures for the ending. My working on the ending is in no way indicative of my progress through the song, by the by. I just happened to have noodled with a melody that I felt would sound good as an ending. At any rate, in measure 1 and 3, there is literally a single note that is interchanged between the two, and only by a single tone. And I couldn't for the life of me decide which I preferred. The change is so slight that when I sent it to a friend for an opinion, I was told, "I listened to both samples a few times and can't tell any difference," which makes me believe no one but a professional musician or someone with a very refined ear would even care, so why am I tearing my hair out about this?
Besides, I've decided that I need to try and treat song writing as I would a painting. A simple base layer to start with then slowly and steadily adding more and more details until it is full of texture and, by the time you are finished, becomes interesting. Hopefully.

I'm only now coating the base with color and am a long way from painting any sort of sunset. I'll try and get the colors right when I get there instead of worrying about them while it is still early morning.