PLEASE HELP

My pretty little girl, 18 months old now... is still biting. she started around 11 months old and hasnt stopped, and she knows she isnt suposed to. i have tried EVERYTHING. from time outs, to spending more time with her, to socializing her, to working on other avenues to vent when she is angry, tried to get her to kiss instead of bite, explain to her it hurts and is unacceptable, i make her say sorry to whoever she bit, and she does, she also plays the kiss not bite game with me, but she just randomly BITES people, particularly other kids. older, or younger. and she bites hard, draws blood, i dont know what to do. a friend of my roomates came over with a girl her age, we let them play, they were doing fine, then just out of the blue, she walks up to her, sticks her finger in her mouth, and bites, HARD. i had to pry her off the other kids finger. the finger swelled up. then i find out they were asking if she had shots cause they wanted to take her to the hospital to get her checked out cause the bite was so bad. today i had a friend over for dinner with her younger baby and 5 year old son.
she bit both of them. she was playing in her room wth the boys and bit them. i made her say sorry. she did and they played for a while longer without problems. she did bite him again later. we had dinner, she was good, ate her dinner, even got to sit on the big people chair and be a big girl. the kids were all in the living room. the baby, was crawling on the ground playing with a toy. we all had our heads turned and hear a scream.... the baby was crying and my daughter was standing next to her looking guilty. there was a HUGE bite mark on the side of the baby's face. swelled up, baby kept crying. i immediately sent her to bed. no ba ba. no bedtime songs and stories, NOTHING. just in bed, lights out, end of story.
I FEEL HORRIBLE. really? what can i do to make her stop. ive tried everything anyone has ever told me. YES i have bitten her back, Yes i have even smacked/flicked her pretty hard in the mouth, yes i have tried other avenues, yes i try to spend time with her, we spend 3 uninturrupted days together a week. i try to take her to parks, play with her. those are her days. she is with me and austin only pretty much now, we changed our schedules so she is not with babysitters.
why is she doing this. im going to see a child phsycologist or something. im really upset about all this.