What should i do in this situation?

boyfriend won't defend me because he says it's my fault his ex hates me bc i don't let them hang out. i tried to befriend her in the beginning and i tried to give her a chance because i didn't want to become the controlling girlfriend but she would request to hang out with him alone and withhold things from me that only he could know. she has also threatened to commit suicide 3x and now my boyfriend is stuck paying her ambulance bill bc he's the one that dialed 911 which is complete bs. she also has a bf yet she continually texts my bf to hang out. my boyfriend and i have been dating for a year and a half. she took his phone and sent me messages calling me a fat **** and a ***** and saying things like she hopes we break up and i think you are ******* putting your gf before me again on the night he told me he was going to straighten her out. while i don't think he cheated on me i don't think it's nice that he didn't defend me. i tried talking to him about this but he said he doesn't care to do anything about the situation and he still hides the fact that she's in contact with her. he'll also tell me he can't help the way i feel about this situation and that i live in a fantasy world. idk if it's because he feels sorry for her and idk if he's necessarily over her or if he is emotionally cheating on me with her. I should also add that they dated for 3 years and they broke up in 3 years ago bc she left him for one of his friends. i did try to be cool with them hanging out but i found it highly annoying that he'd go chill with her 3x a week alone plus at mutual friends parties. it also weirded me out that shed text him about how she had bruises on her body bc shes anorexic and that she texted him on a separate occasion to tell him how things werent working out with her boyfriend and she needed him to come pick her up.