Stuck in an abusive relationship and I can't get out?

My S/O is extremely emotionally abusive towards me. He freaks out about smallest details. He calls me a slut, a cunt, a bitch. Accuses me of cheating. Won't let me have any male friends. Makes me delete old photos (selfies) from years ago. Will ignore me for weeks if I do something he doesn't like. He also talks about wanting to hit me really hard during sex and wants to see me cry because it's "hot". He's done a complete google check on me and will make me go through and delete things he doesn't like, like old twitter accounts I made all the way back in high school (there's nothing bad on them)
I can't leave because I feel like when times are good, he is one of my best friends. He's the only person who is there for me to talk to. He's done romantic things for me no other boyfriend has... But that's only when times are good.
I'm scared. I love who he is when things are going well but he drives me insane when he gets this way. He is currently making me delete an old social media account I no longer have access to anymore and is refusing to talk to me until I do it. He knows I can't and told me to figure it out. He's also now threatening me with an intimate video the two of us took. I know it was stupid to do but it was before I knew this side of him. 
I know I have to leave him. I just can't find the strength for some reason. I feel like I'll be leaving one of my best friends and it kills me. Please don't bash on me, I KNOW I HAVE TO LEAVE. 
*sigh* I'm well aware he isn't my best friend, the point I was trying to get across is that I stick around for the good side to him. Unless you've been in this kind of relationship, I don't need you to tell how easy it is to leave, or to be making fun of me?! If you can't act like a mature adult on here then please just don't comment. I'm aware I need to leave, if you read what I wrote you would know. The point is on scared and he's been threatening me. They aren't excuses, I'm seeking help from women who have also been in this situation. If you have nothing nice to say don't say anything at all.