Moving on from someone you loved is a constant battle. 
It physically and mentally drains you because you're constantly fighting back memories and feelings. And that's why at 2 am when you're all alone, you become weak. You're so tired of fighting it. And then the memories come back and the longing returns and suddenly you want to pick up the phone and you want to be in their arms, but you don't do it. Because in the morning you'll have to go back to being strong again and you'll have to go back to fighting it, and a phone call wouldn't change anything anyways..