Ex vs. Boyfriend

I have a bf for almost 8 months now. However, I have noticed that our long distance relationship it's affecting our relationship since we begin to argue for any unknown reason. For that reason I feel things have chance between us a little bit. However, in the meantime my ex( who happen to be also my best friend for years) contact me since Christmas. He had text and call me since then. I have responded and we have catch up. However, I'm really respectful since I have a bf plus I want my ex respect my relationship as well. The point is he keeps calling me because he said he can't forget about me and don't want to lose me. I really don't want to lose him as well but I have a bf now and don't know what to do. PLESE HELP!