Cheating

How do I leave my boyfriend who i have a beautiful little girl with she's almost 4 months, and have been dating for over a year and 5 months. And have been living with for a year and 5 months. I need to leave him but I can't bring myself to do it as I don't like being alone... I'm 19 and have no family of my own..... I'm leaving him because while I was pregnant he got "bored" with me, one night I had one of my old high school friends over and he asked for a threesome he said yes I said no but decided to do it to make him happy.. after the threesome she started spreading rumors that I'm disgusting, started inviting my boyfriend to stuff and not me. Just texted him all the time and not me and that she would make a better mom to my little girl. Deep down I knew he did something with her other than that night.. at exactly 37 weeks pregnant I told him a cheated which was a total lie to get him to admit it. He told me that night that it was okay because he also cheated. I than got very angry and questioned him because I blurted out "I knew it, I never cheated, I just wanted the truth. He than cried and I walked away.... ever since he doesn't want to talk about what happened. Won't assure me anymore.. and we have fallen out of being in-love. I need to break it off but can't have tried but haven't gotten far...