A general way to overcome this condition doesn't exist, since every person is different and the circumstances, which influence each person's life, as well as the emotional situations vary.

You seem to be over focused, which inhibits your creativity. You look at the end of the creative process and wish, you had already created your song without knowing how it will sound like, instead of going through the creative process (the creative process is really satisfying!). In my opinion there are only few rare cases in which an artist snaps his fingers and everything is done. You have a particular outcome in mind, but can not write it down, it could be, the outcome in your mind is a problem, that suppresses real creativity.

That said, here are my two cents:

1. Get rid of the fixation on the song, distract yourself. Push minor creative projects, that come easier to you and enjoy the process of creating something. Have fun!

2. If you feel like it, get back on the song and look at it from a distance, as an objective observer and tell yourself, if it looks from the outside as promising as you felt about it. If not, discard it.

3. If you still like the project, play with it, goof around. Do the opposite you originally intend to, set themes to go with, a dark voice speaking, a child involved, what wold it sound like if you mix it with a little yellow, etc.

4. Consider writing the song in more then two or three steps, like an important side project. Maybe that is the technique which fits you and your song best. You don't have to write the song in one or two brutal sessions.

5. DO NOT COMPARE YOUR EMERGING SONG WITH THE ONES YOU REALLY LIKE IN THIS MOMENT! This would guide you in a specific direction (which can be fun too, btw) and makes your song generally a kind of remake or absurd mix. Your goal is to be creative, though that doesn't mean, that your cultural background isn't the source you use to create, and which sets the boundaries for most of your creations.

TL;DR: Don't be too focused on the project. Distract yourself. Step back and look if it is worth it. Play around with your project, maybe it loosens the tightend knot and gets you somewhere.

Edit: Some spelling.