Why cant I stop thinking of this guy (please tell me something)?

Okay well, I dated this guy my freshman year, till my sophomore year, where I broke up with him, cause of my mom.. We stayed friends, time passed by and we lost contact until this year we are both seniors, and we started talking again in the beginning of the school year, after he added, and commented my picture on fb. He gave me his number and we started texting, he would reply back with one word answers, one word sentences and I realized that I still had feelings for him.. I wanted to tell him, but I was afraid of rejection, until I decided to tell him what I felt inside, I wrote it in a text, my sister grabbed my phone and sent the text, as I was going to press cancel.. He replied telling me that he no longer felt the same way he did for me before, but that we could stay friends, things werent the same after that text we stopped, we stopped texting once again, and now we see each other in the school hall way, we lock eyes, and we try to play it off.. I want to know why I cant get over him, like I did with my previous ex boyfriends, why am I still stuck on him? I know I have to move on but its difficult. I’ve tried dating others, to replace him, but I stil feel this way for him..