sunflwr23,
what you have to do is start leaving your emotions out of reasoning.
This is basicaly what is happening: your brain still makes connections to happiness and being with him. Therefore you feel incomplete and unhappy. In other words, your brain hasn't accepted yet the relation is over (aka hasn't rewired itself).
This influences your thinking (reason) in ways that it makes you unhappy, that you are holding on to the past and that you have a hard time moving forward.
Awareness of what is happening on a biochemical level, will make you understand better why you feel the way you feel and think the way you think.
Now the good thing is that you consiously can 'rewire' your brain to associate happiness with something else than your ex.
This is called brain plasticity or neuroplasticity. I suggest to read up on these keywords, and I am quite sure you'll start understanding how what you think is how you feel and how you can overcome this just by slightly changing the way you think.
Hope this helps.