How do you let go when you love them?

I've been engaged for 2 years, and with my fiancé for 3. I can honestly say this is the only man I've ever loved. You won't understand why, after reading this post, but for some reason, I do love him. I didn't mean to fall for him, but I did. He used to be sweet, and fun, and surprise me with flowers and love to spend time with me. I've caught him cheating on me numerous times. One time I caught him and was so upset I threw my hair brush at the wall, and he choked me. To this day he says "I shouldn't have acted like a man and thrown something at his wall and I wouldn't have gotten treated like a man." He thinks he had a right to do that. So I got choked because he got caught cheating? I don't understand... As many times as he has cheated, I could never do it to him. Two wrongs don't make a right. And in a weird way I feel as if I have to mother this thing. He's sensitive, when his friends say something about his weight or anything offensive, I feel like I have to protect him and have his back against them. He stays on Facebook constantly talking to other females, he even got fired once for it. I got a job as a waitress not too long ago, and he said it was "petty" and "not a real job" and that if it weren't for him I would live in a cardboard box. And other mean things. He left me on New Year's eve to go to a party, I spent the New Years with my cat and a bag of beef jerky. And what scares me lately is the cheating doesn't bother me. It doesn't make my heart drop when I see it or think about it. It doesn't make my eyes tear up. I don't even care where he goes or what he does anymore. I'm finally over it. But at the same time I feel like he needs me. I feel guilty for wanting to let go. I have half of my things packed, but I don't know how to tell him that I don't want this anymore. I want the best for him, and happiness, but loving him is so toxic to my health. I picked up smoking because of the stress, I always feel ugly because the girls he has cheated with are way more beautiful than me, and I even got an std from him. I've gone from 155 to 120lbs. I want to be happy. And I want to be loved as hard as I love! And sometimes I feel as though these wishes don't come true.