Relationship Issues. It's killing me.

My Boyfriend and I have been dating from almost 3 years now. He would do anything and everything to be around me and do anything for me. But he would always flirt with other girls. Slowly, things started Changing since the time his Parents started disliking me because I come from a Divorced Background. And then his mom was diagnosed with Cancer and that time his parents asked him to choose between them and me. So we had to call it a break. But technically, we were still together. Everything was normal. Then he started talking to this girl from Feb, & since then I had my doubts on them. Months passed & in the month of May I found out he was cheating on me. I tried to ask him normally about it. He said she's just a friend. It broke me up completely. After months I took the courage to confront him and caught him red handed. He cried & begged for me to come back. I gave him a chance. But he still talks to her & lies to me. I love him a lot to loose him. Plz help. It's killing me.