Who should i believe? my boyfriend or his ex girlfriend?

Me and my boyfriend have been dating for 4 months now, about 1 month of dating his ex got in our lifes spreading rumars around saying that he is cheating on me with her, but i didnt know who to believe, so i took a chance and i believed my boyfriend, he told me that he was tired of her and that he couldnt stand her trying to miss his life up again, when they were dating she made his life so miserable, breaking up with him then him crawling back, she treated him like a puppy on a leash that would do every comand for her 4 three years this went on, but then the last time she broke up with him and he tried to get her back and then he meet me and didnt want her anymore. and she got mad i guess and now i dont know what to do, on sunday i got some letters saying that he didnt want to talk to her anymore letters he gave her, and one letter she gave him back said that she just wantted him to leave her alone. and as far as i know he had been leaving her alone she is the one that comes up to him and screams at him saying that she cant believe that he is dating me and everything. I am so confused on who to believe anymore, should i believe my boyfriend or his ex? plz help me best answer gets THE BEST ANSWER ='(