Complex sentence: Ross would eventually help Carrie who then developed a third personality, a childlike one named Carrie MacKenzie, but shortly after Carrie would leave Springfield to seek further treatment in England.
Split into simpler sentences: Ross would eventually help Carrie get treatment, who by this time had developed a third personality, a childlike one named Carrie MacKenzie. Shortly after Carrie would leave Springfield to seek further, more extensive treatment in England.