What do I do? I don't want to leave but I'm being constantly hurt...

This is something of a rant but...My fiancé is abusive. He grew up in an abusive home, and now reacts the same way to situations as his father did. [TW for the next two paragraphs]

It got really bad last year, when for several weeks he would choke me while threatening my life. That's when I realized there was something wrong. It eventually culminated in mild sexual abuse when he kept me in the bathroom for half an hour, then spent fifteen minutes slapping/playing with my genitals while making comments about why I was turned on. It ended when I moved out for a week or so.

It calmed down until last November, when he started to abuse me again and he's kept it up for the last several months. For example, when I asked him to pack his lunch in his backpack this morning(which I'd made him) he forgot. I got a speech this afternoon about how I'm fucking useless, a heartless cunt, etc. while I raced home to get it. He's pulled a knife once or twice. Yesterday, I failed in finding his phone after he went to sleep the night before and he kicked the table around. I ended up freaking out, and screaming, and he clasped his hands around my mouth; I crawled away as fast as possible in terror. He calmed down, but made me pick up the shards of a jar he broke.

Anyway, I just really don't know what to do. My fiancé knows he's abusive, but he keeps doing these things. I want to stand by and love and forgive him, but he keeps hurting me. Hell, I'm developing worrying signs of flashbacks from last year. I'm constantly on edge around him. Anything I do wrong is ruthlessly picked apart.

Just...what the hell do I do? Can anyone sympathize, or offer advice? I just feel so lonely in this, and like this is all my fault. And I don't have a real good measuring stick of what a healthy relationship is since this is my first.