Help! What should I do?

My husband has been texting a female co-worker for six months now. When I initially found out about it, last September, I explained to him that I didn't feel comfortable with him texting other women unless it's work related. He told me, he understood and said he would stop. A day later, they began texting through a different texting app. I didn't find out about it till one month later. They would send a few, what I would consider flirty messages and selfies to one another. (Fully clothed) I was so upset when I found out. I told my husband he had to stop or I would eventually leave him. He promised he would stop. Anyways, I just found out they continued texting one another, but he had been deleting the messages. I feel like I'm at the end of my ropes... He claims they're just friends and the pictures they send to one another is just a game- nothing to be concerned about. He keeps lying about so much, I don't know what to believe anymore.