Why does this guy pop up in my head? I only dated him once, long ago?

your not going crazy i can tell you that but you might of had some special relationship with him that you havent ever had before but now that you are completely moved on you might of looked back on your memories and thought about good experiences that you had with him and you might not be able to get him out of your head in the momment and thats it just try and do things with your friends that will hopefully get your mind off him hope this advice helps... Hope this helps (p.s. you went all out on this question lol)