Why do men lie and cheat?

My baby daddy and i were together for 3 years. I brought us a house so that we could raise our child together. He told me that we would get married after we got the house. About a month after we got the house he started going to the clubs and drinking on the weekends which was something he had never done before. About a month after that he comes to me and tells me hes not happy and needs some space. So I told him to go take his space and he said he would be back in a week. Well over that weekend I went to see him and he told me we needed to work on our communcation and trust issues. About five days later he comes home with no bag and tells me that he doesn't feel comfortable at the house anymore.When I ask him why he just looked at me so i asked him if he cheated on me and he said yes. I asked with who and he told me who it was and that he had feelings for her. Within 3 months they got engaged and within a week they were telling each other they loved each other. Now I feel so hurt and still can't believe this has happened. Do I have the right to be angry? How long will the anger last?