I feel like I'm a rebound and that he's not over his ex!

My boyfriend broke up with his last ex-girlfriend almost 2 years ago, I'm not very sure he is over her and I feel I'm just a rebound whenever I ask him questions about his past relationships he doesn't get frustrated about any of his exes except his last ex of 4 years, there is always something whenever he talks about her , he says he is over her because she told him that the 4 years they spent together was a mistake, it feels to me that he is hurt but is still in love with her