I've been with the same guy for three years

My current boyfriend, I have been with for 3 years. I met him when I was 15, and started dating him about a year later and I'm now 19. Sometimes I find myself questioning if he's really "the one." And I'm not saying this because he is bad to me. He is close to perfect, treats me like a princess, is there for me whenever I need him, I could go on forever. I've loved him like crazy ever since we got together, but for the past month I've been questioning if I just love him or if I'm IN love with him. He's the first serious boyfriend I've ever had. We live together, and had talked about ttc within the next year or so, and getting married. 
But sometimes I just don't know what I want anymore. He tells me he can't imagine a life without me, and that makes me feel even worse. I love him so much, but I don't know if I want to be with him anymore. I haven't felt like this for a long time so I want to hope that it's just a phase. Your opinions?