For me, as I learned the guitar parts of the song, I hummed the melody and gradually converted the humming to the words of the song.
The first song I learned to play & sing was Cat Stevens "Father and Son". It took about 2 weeks of daily practice. If things are not going well on a particular day, give it a break, try again later and give yourself the mental OK to quit for the day if things are going really bad. No sense in torturing yourself.
Ray
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