My boyfriend being untrustworthy?

Ok so ive been with this guy for a month yesterday. Weve been good friends for about a year and have liked eachother since we met. So I really care about him and love being around him but recently a "'friend" of mine moved in next to him and theyre constantely together. I confronted him and he swore hed stop. Lied. They still spend a lot of time together and she flirts with hiim infront of me constantely, Ive told him repeatedly I dont want him hanging out with a lot girls or the relationship will be over. He promised hed never do anything to mess it up.. Well his sister is just one grade up. One of our many school skanks is her friend who is her age but was held back... She spends the night at their house a LOT! I did my best at trusting him up until an hour ago when I found out hes going to her house this weekend and kept it from me.I like feel like he deserves a punch in the face. WTH!? What am I suppoused to do about this? Im like so po and am on the brink of bawling my eyes out. I love him but dont know if i want to be with someone like that... Help me.