Complex sentence: He also is overly emotional, easily breaking into tears or getting overexcited at critical moments, to the point where the quality of his singing is affected by the state of his relationship with Eiri.
Split into simpler sentences: He also is overly emotional, easily breaking into tears or getting over-excited at critical moments. A good gauge of his over-emotional state can be seen when the quality of his singing is affected by the state of his relationship with Eiri.