Do ya think i should dump him and leave him and his dramatic ex behind for good?

My boyfriend let his ex overstep my boundaries and tried to use the excuse that she was like a sister to him. But I didn't fall for that because I can read her like a book, she's manipulative and suicidal and she runs to him about her problems like a child and has stated "I feel like you're ******* putting your gf before me again" to him. Of course I should spend some time with him! He not only has work and college but band practice and war machine tournaments. I am also a pre-med student and I only come into town like once every 2 months while she used to hang out with him 3x a week. About a week ago she took his phone and started texting me saying that I'm a fat **** and she hopes we break up and a bunch of other nonsense. She even tried to get me to fight her, but I called her a loser and told her I didn't want to waste time with her because I am a busy woman with more important things to do than discipline a child. Well my bf promised me he wouldn't have anything to do with her anymore after that episode. Well now I'm in a little bit of a rut because I just found out he owes her $1000 for calling an ambulance the last time she tried to kill herself. I had to weasel that information out of him because I had a gut feeling something was up, not to mention he kept mooching off of me when he works at least 25 hours a week! He tried to say that would have never happened if I had let her come chill with us at a get together that night, but I know it's not my fault because she loves to start drama, I cannot trust her and I'm a college student that doesn't have time that pathetic waste of life. On top of that I'm sure she can make her own friends and find some way to occupy herself that doesn't involve my boyfriend or starting drama with me. I heard she has a new boyfriend that she started dating last month, but again she would run to my bf to complain about him too. My response to my boyfriend was that I should have threatened to kill myself ten times over now with all the real world and adult issues I have going on in my life (in a joking matter) such as volunteering at the hospital, taking organic chemistry, synch swimming practice and the fact that my father and dog both recently passed away. My bf and I are both 20, and she is 19 but she failed to graduate high school the first time around. What do you say I should do about this? I know better than to trust my boyfriend bc he deletes his texts and phone his phone history and hides his phone and he has lied about contacting her in the past.I want nothing to do with her because god forbid she tries to call the cops on me or some childish **** just to get me back and ruin my life. Am I just too mature and should I just start dating 25 year olds?