hmm... if i understand this correctly.... your writing these lyrics to hopefully get your girl back. you need some help in knowing what to write so that you can please her or win her back... also your somewhat fucked because the programs you use to record your vocals on, just aren't working... therefore you want someone else to spit those words for you.... you then will burn the track... and deliver it to the chick that broke up with you....
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aight... i don't spit... so i couldn't record that for you... and also iam thinking that your ex-girl wouldn't be to cool about speaking the words that came from your heart.... she'll be more emotionally attached if you flowed those words yourself in front of her accapella style.... no beat... just you..... your heart...and words...
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as far as co-writing.... can't help you on structuring the song into rhyme-form...... but maybe you should bring the most important good times you had with your ex-girl... that should at least have second guess herself.... into wanting to go through those experiences with you again.....

hope that helped...