What do I do about my ex?

So I have a bf and all and I love him to death. The problem is my ex. I still have these overwhelming feelings for him that I can't seem to put away no matter how hard I try. He was my first real love. I keep thinking about him and today he kissed me and I don't know what to do about it. I feel like the world's most terrible gf ever bc I liked it and I kissed him back. I still haven't told my bf. I'm scared that if I do, he will leave me. What do I do?