Hey
I decide what emotional space the song exists in. If I don’t have exact experience, I use an imagined equivalence. For example if I want to write about the devastating loss of a child, I can use the devastating loss of a close friend, other devastating losses, to begin to approach how that might feel.
Once I feel the emotion, I sing melodies (usually) while thinking about the subject of the song. I step myself in the feeling and try to express it in purely musical terms. If I have pencilled-in lyrics, I perhaps try singing them.
I always switch to melody as soon as I can in the writing process. I try to view any phrases I have before that as fuel. They might make it to the finished song (though not always as consecutive lines if I have more than one), but I am completely prepared to ditch them.
Initial words and phrases are but the seed that a song grows from. For me, my songs tend to be limited by attachment to draft ideas more than any other factor. The attachment hems me in. Over the years I have learned to let it go by deliberately tackling it.
You seem to be limited by not having distinct writing phases. You are mixing the draft phase with the edit phase. That means you are qualifying ideas as you go... which sucks as a way of generating ideas and really, really slows down your writing process. This probably has an effect on all your creative tasks. It usually stems from an over concern about mistakes, every decision becomes critical, everything needs to fit a growing list of criteria before you will accept it... coupled with an over attachment to preliminary ideas.
My old art teacher had a novel way to address this, and an equivalent solution can be found in songwriting.
Artists who suffer from this often hesitated in the marks they made in a page. Often their drawings were rigid, and because they focused on detail prematurely, their art often had skewed perspective, the relationship between sizes was wrong.
The hesitancy came from feeling every mark on a page was critical. Chiselled in stone for all eternity. So they couldn’t make a mistake.
The solution was to pin 10 sheets of newspaper (the cheapest printed paper, already used and ready for the trash) to a drawing board, and give yourself 1 minute, or 2 minutes to sketch what you saw before you. After the agreed minute or two was up, you tore the sheet off the board, scrunched it up, and tossed it in the trash. The exercise was not to come up with a finished art work. Sure it might give you an idea, but this was not the implementation of grander visions. It was to get into the mindset of sketching what the mind saw quickly, with zero attachment to result, zero qualification of ideas, no long term buy-in.
Quite quickly artists cured that hesitance and over qualification and premature attachment that stifled their work. They learned to express themselves.... and that is one of the key things we need to do as songwriters.
I would recommend the equivalent approach for music. Try random mood selection using 2 dice, write a melody in 1 minute or 2 minutes (work up to 5) that matches the rolled mood (2 equals sad, 3 happy, 4 excited etc.) At least at first try recording the melodies, then when the time is up... delete them. Roll dice, next write.
When you go back to your normal writing, try having a distinct ideas and draft phase, the equivalent of sketches. Don’t go to edit phase until you have a draft of the overall song. Know that edit can be done in cycles (try an idea, if it works keep it at least for now, if not ditch it, at least for now) you can re-enter draft phase every time around the edit loop. Get used to knowing ideas are not fixed in stone, and at any point you can go back to trying new ideas at zero cost... after all, you still have the previous version of the work so, nothing ventured, nothing gained
Cheers
John