Why did he cheat on me :(

My finacé and me have been together 3 yrs. 3 wonderful years. But he NEVER cheated on me until now. He proposed to me in September and cheated on me December. He cheated on my 3 times before he told the other girl about me. He apologized immensely but he’s just now telling me (March). Should I believe him that he’s sorry? Since he actually told me about his cheating