thinkin about during sex.

My boyfriend of 5 years(on and off) just told me that recently during sex he doesn't think about me, he thinks about his ex girlfriend. 
I don't even know how to react. I am so many different emotions. 
When he admired that he told me I deserve to know since "I'm so special to him, and he doesn't want to lose me." So I asked him if I even turn him on anymore, or if he finds me attractive, and he said yes but he never slept with her so he wonders what it would be like. 
To me it seems like he's not interested in me anymore, but I don't want to admit that our relationship might be over. What would you do?