I don't know how to finish a song I start. How do you do it?

I've two goals in life:
1. Have a movie credit

2. Write, perform, and release a song

I've been writing a lot longer than I've been on this site. It's never bothered me that I struggle to finish something (a story, a song, whatever it may be) because other people aren't going to see it. Then I jumped on some battle sites, stuck with this one, and started sharing what I was writing and seeing if people responded well to it. There's been a dramatic improvement from my day one to where I am now, if anyone's read my early>late stuff you can see that yourself.

But now I want to be able to record a song and be satisfied with it. I don't have dreams of being a rapper, of being famous, of getting a following...I just want to create art and be proud of it. But I can't stick to a theme. I don't know how to maintain a flow over a beat. There's a beat I've got that I fell in love with, and it started off pretty well. It played probably 40 times and each time I was adding more and more to it. But I couldn't put enough RELEVANT AND QUALITY content in between each switch. This has been my problem with any beat I've tried to rap to, but this one annoys me the most because I felt like I was getting close.

I've revised it probably 5 times and I can't fix it. I keep adding new stuff, removing old stuff, changing the new stuff, bringing back some old stuff. Hell, it started as just me, then two characters talking (rapping) to each other, then the characters being a guy and the voice in his head/mental illness talking to him, and back to just me. My favourite version of this, was the idea of Me rapping as the guy and I had Nigma's voice in mind for the Voice in his head. 

At the moment it's almost the perfect length for the beat, but it's so random. I feel like I've taken too many concepts and ideas in an effort to fill in the gaps. Now it just sounds random, like every few bars should be part of a different song (perhaps that is the answer...but then I'd just be losing 90% of the song). Some lines are from past verses (and some reworded), but I thought together they would make for a good track...now I feel like I was wrong. Most are new.

ANYWAY. My concern is that I don't know how to stick to a theme of a song. It's all within the same realm, but it just feels like it should be elsewhere. I'll put the lyrics in a reply to help you understand what I mean (and so this giant wall of text isn't bigger by default).

Tips? Advice? Anything?

Thank y'all.

im free, free, man im free                                 this part 

yeah im finally free is an intro with

from my morality       a very slow beat

i got no idea what lifes got for me                            basically talking to myself

but I guess we'll see

as i go on this journey

hey Mem, hows it goin'? (wait who th-?)  brackets are "the guy"

watchya doin'? (how did you-?) no brackets are "the voice"/somebody else

still cashierin' drawers (yeah?) and moppin' floors? (i mean yeah bu-)

cshh, man, aint you meant to be tourin'? (it aint like tha-)

wait, didnt you say...

//////////////////////////////////////

I ain't been trapped in the lab, or locked in the booth     rest down is all "the guy" until the bridge

I just got the gift of the gab, rappin in my room

This loathsome life is so drab, fadin in my tomb

If i die you'd be glad, im stayin on the move (OOF)

//////////////////////////////////////

Yeah I make stupid decisions

I once had vivid premoninitions of my future winnins (HAH)

But they turned to convoluted visions from my brain symptoms

But I ain't just another victim of this sickness

//////////////////////////////////////

Treated like a nuisance, no excuses

So deluded, feel so useless

So many screws loose, maybe the noose is (CUT IT OUT!)

So stupid for thinkin' Cupid shot and miss

I'm over it, sober and hopin' that his arrow hits

//////////////////////////////////////           all the "////////" were originally to

Horror scenes that haunt my hopes and dreams separate the characters, its not

Blood gurgling murder screams, these ghosts have seen currently fully set up like that but

Redrum curdling, they paint the scene they should give you a good idea

Hear the dead hum hymns of the Elohim of what i was going for

//////////////////////////////////////

Sharp razor blade weapons, down the road I test 'em

All my friends and all my brethren

Always guessin' never askin' questions

Fake connections, now I'm gone, with no direction

//////////////////////////////////////

And I tell the therapist I'm always right

Resort to violence all my life

Distort the voices, know their lies

And hope to be immortalized

Through your mortal eyes (suicide)

All these poltergeists torment my mind

(WHY) Yes you can 

(I) Yes you can 

(MIGHT) Yes you can  BRIDGE --- not sure if it should start at "therapist" line

(DIE) No you can't  or if it should start at "(WHY)"

(I) No you can't  atm, the beat changes at "therapist" 

(MIGHT) No you can't  to a slower beat you might talk over

(SUR) No you can't 

(VIVE) Yes you can

//////////////////////////////////////

Misery loves my company

A faded memory

Drunkenly arguing with family

The ones I've wronged, you see

//////////////////////////////////////

Take me back to a time with bliss

Give me back my innocent ignorance

With my friends, misfits doin' stupid shit

But I don't miss the slit wrists

//////////////////////////////////////

I think I'm the worst, I think until it hurts

'til I murmur words that barely resemble a verse

I twist and turn, this intelligent curse

Soul search, heart empty and disperse

//////////////////////////////////////

well bro, i'll keep it real with you, I aint reading all of that.

what you're asking is impossible to answer correctly, the reason being.. everyone has a "different" writing process. you NEED to find YOUR PROCESS. what works for me, may not work for you. I hope others will chime in as well, but generally, what I like to do (doesnt always happen...) is find a mood.

I play a bunch of beats, until my mind goes..yeah..I wanna write to that. a dope beat is a dope beat..but finding your mood.. helps the song write itself. you're probably hindering your own writing by "forcing this"...put that beat away for a couple weeks, write to something else and come back to it. maybe the spark will fire.

once, I have my mood set.. I just start freestyling..finding the "type" of flow I want to use.. occasionally, i'll start my verse on accident :P..here and there ill do the hook first.

but once you have direction..just stay on it..if you find you start to drift..just scrap those parts.

say your song is about your morning routine.. and you get to the walk out the vehicle part..then you just start going on about how nobody raps better than you and youre fucking everybodies bitch..well..we got off course.

one thing to remember when you do this..dont get so wordy...dont cram all the words in. you want to say everything in the LEAST amount of words. that should be your goal. try and trim all the fat.. all the "the's, and's and but's" sometimes really cause you to rush and fuck everything up. see if you can reword bars..even once you're done. you'll look back and go "ah fuck..I shoulda said it this way"

bottomline, dont force..let it come natural or the shit will suck. best of luck AA.

find a beat you could really vibe too. 

Find inspiration

brainstorm and mind map 

try and stay on topic.

either use the verses to sum up a hook

or the hook to some up the verses 

I don't know there's so many ways to go about it. An equal question would be how to not complete a song. There's infinite ways of doing it and everyone's different. Some people freestyle like 4 bars at a time and just keep the shit they like. Some people basically write poetry and just put the stanzas together like puzzles over the beats. Most people put on a beat and write to it about what it inspires them to write about. I'm exhausted right now but hopefully I might of said something helpful