How to keep myself from going back?

Tonight my(now ex) boyfriend lied to me for the third time this week. We were in his bed and right as I was about to fall asleep, he told me he needed to shower. Then he went to the bathroom and called a girl. He turned the shower on bc I guess he thought I wouldn't hear his conversation over the water, but of course I heard everything like their plans to meet tomorrow. To make matters worse he had her on speaker (his phone is faulty so he has to do that to hear people) so I heard her voice and all. I asked him about it when he came out of the bathroom and he proceeded to claim it was a guy he was talking to. He said, "here check my phone, it was Butch." The most recent call on his phone was to Butch, but that conversation had occured hours ago, so I knew he deleted her off his recent call list. I left his house pretty upset, and later he called and admitted the truth. He told me she was just a friend, but if thats the case, why lie? This is the third time hes lied to me this week, and the second time its involved a girl. We have 2 college cles together and its really hard to think about seeing him nearly everyday. Does anyone have any advice of how to remind myself of this pain I feel now, later? Like, I know I will get over this pain, but im afraid once I do I will forgive him and take him back, making me vulnerable to his lies and susceptable to being emotionally hurt all over again. I really want it to be over for good this time, but im easily persuaded and I loved him a lot. I fear I will go back to him. How do i stop this from happening?