Tunes aren't fixed, they adjust themselves to match the words used with them, and the same happens the other way round

English verse doesn't operate on the number of syllables in a line, but rather on the stresses in the lines. Lines with very different numbers of syllables can go with the same tune. It happens in the different verses of songs all the time.

If the tune sounds right when you sing the song, it is right. Of course it might not be exactly the same tune as in the song you took it from, or in the tune as it is written down in musical notation in a book, but so what? You learn to play what goes with what you sing.