how come we dont see how much someone means to us until theyre gone?

i dated this boy 3 years ago. we were on and off for a year. i was always the one to break up with him because i liked someone else. the next year we didn't talk much. then he gave me earrings to make up for not giving me a Christmas present the year before. we started talking again and we became best friends. we could talk about anything and he would always listen to me. i knew he never stopped liking me and i kinda liked him but i was afraid to go out with him because people made fun of him. he used to be popular but then started hanging out with different people. But anyway he could always make me laugh and make me forget about things when i was upset. we stayed best friends for a while then we went back out a year ago. my friends gave me a lot of shit for it and stupidly i broke up with him.... again. and still after that he wanted to be my friend. he told me that he never wanted to lose touch and wanted to be best friends forever. he was really sweet and the best person Ive ever known. Then he moved. he was okay for a while and we tried to make plans to hang out. but one day i called him and i noticed he had changed. he was meaner he didn't really care about anything and he kinda became a typical jerky guy. i had just realized how much he meant to me and that no one else could compare and now he didn't feel the same way about me anymore. i had 3 years and i messed them up. My ex best friend claimed she liked him and made plans with him right after i told her i still liked him. I'm not friends with her now for a lot of reasons. him being one of them. i realize i kind of deserve it and I've tried talking to him. I've tried to tell him how i feel but hes not sentimental like he used to be. it hurts so bad and i want him back. i want to make things better but hes moving soon.farther away. and I'm afraid if things don't change soon.... they never will. and i really don't want to lose him forever.