thinking of first love

Im married but iv never missed my  first love. Btw weve never had sex as he respected me. He got married to someone else and so did i for our parents sake. He was lovely.
 But recently after 15 years of marriage .. hes just come to my mind all of a sudden.  I cant stop crying these few weeks .. 
why am i feeling like this its not normal i dont want to think about him but he just pops up in my head and i start to cry. I try to keep myself busy but its difficult. Hope it passes ...