Why am i so confused when it comes to likin guys?

How come i'll really like a guy until i get him then i don't want him anymore and once he's with another girl i get jealous? i don't understand it! and how do i stay happy and content with the guy i'm with and how do i not be jealous? and how do i move on? i'm all mixed up! whats wrong with me? aaahhhh!!!