Complex sentence: Lisa convinces Bart to give up on Mrs. Berrera and let Seymour have her as a girlfriend, but when he presents her to Agnes, she gets so disgusted about his mother that she decides to end her relationship with him.
Split into simpler sentences: Lisa convinces Bart to give up on Mrs. Berrera and let Seymour have her as a girlfriend. When he presents her to his mother Agnes, she gets so disgusted about his mother that she decides to end her relationship with him.