I'm so sorry that you are feeling like this but I think it is completly normal...part of that whole rollercoaster ride they keep talking about. You are going to have up days where everthing feels completely fine then those down days where you can't get him out of your head and you feel like crying all day.
Just stay strong and vent on here!! That always makes me feel a little better. Try to get your mind off of it, go for a walk or pop in one of your favorite movies. If you can call a friend tell them you are feeling a little down and they can come by or you guys can hang out and that will keep your mind off of things for a bit.