I think you are correct in both your examples: "I guess it means the only one who 'can make me feel sad or frustrated' or 'I can't control'."
"And your whole wide world, in a moment, comes undone". This means that your perception of how everything works; how everything fits around you, is shattered. It's a metaphor for falling instantly in love.
Also, its use throughout the song [ Cause the only one I come undone for is you] is probably a metaphor for getting undressed....
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