I am a 22 year old male in an abusive relationship. What should i do?

hi, my girlfriend is 21 and im 22. She attends college and i am a business owner. We have been together since i was 16. Recently about 2 years ago, she started to physically abuse me. The first incident was when i was making us food and because she was pissed at me she took a rolled up magazine and smacked me across the face as hard as she could. I didnt talk to her for days. Another time was when she was in the living room studying and i sat down and turned on the tv to watch the basketball game. she complained that she was studying, i told her to go to the library or to the bedroom because the living room is a mutual living space. She got pissed and threw one of her hardcover books in my face from about 3 feet away. I took her book and threw it in the garbage and then she hit me in the face with a closed fist. Sometimes when were arguing, and we stop she will just walk up behind me and smack me as hard as she can. I can recount over 20 times she has been abusive to me over the last 2 years. Recently she has been more mentally and emotionally abusive, i also suspect shes cheating on me. She would do things like close the door in my face, or when im about to leave she will say "*****" when i walk out the door. One time she asked me if i wanted a glass of water and i said yes. she brought me a glass of water and right before handing it to me she spit in it. Ive never been abusive to her physically or emotionally. shes been this way ever since she started school. right now, as i write this, shes in the living room and i believe shes talking to another guy, i walked by her and she was talking to someone on the phone that sounded like a guy. Im thinking about packing all the things i need and just leaving and let her come home to nothing.