“his heart.” This is now referring to the king of the South, Ptolemy IV Philopator.
“lifted up with pride.” Ptolemy’s heart was lifted up with pride, which caused him not to press his advantage and head north and completely conquer the Seleucids, especially since he still controlled Phoenicia and Palestine. This would turn out to be a huge mistake because the Seleucids grew strong again, and thus Ptolemy “would not prevail.” While Ptolemy was basking in the glow of his victory, and generally not being of a warlike character anyway, Antiochus III the Great focused on conquests in other areas, and gathered his troops and was very successful. From 212-204 BC he moved east as far as India, and as far north as the Caspian Sea. Meanwhile, the Egyptian king, Ptolemy IV Philopator, died in 204 BC, and his young son, Ptolemy V Epiphanes, who was four to six years old (the age is not exactly known), was crowned in 203 BC.