Help me please?

Once before my boyfriend startin textin my ex boyfriend n stuff behind my back, i got reali angry n he promised he would never do it again. However my boyfriend started textin someone else. Ages ago he had a thing wit this gal called stan, after this he told me he hated her n even deleted her number BUT he saw her friend out n got her number n started textin her again askin her to meet up n stuff. I only realised wen she texted him n he tried to hide it. He has also been bein reali two faced, like slaggin ppl off then goin all round them n arselickin them. Wot should i do? X