You don't have to be "strangers" but you should start distancing yourself and seeking out other friends. You can't really be "friends" with an ex like most people think. In the end that relationship causes more problems. Either one person clings to hope to get back together, you end up having "hook ups," or when you find someone new that person can be threatened by the ex, or if he finds someone knew she could consider you a threat. It's all very complicated and human nature tends to drive people this way.

My advice is don't be mean or cold, but do start to distance yourself slowly. It will also help you heal to make new friends and take your focus away from him.