Missing your ex is natural, but do you understand why you are missing your ex? Of course, there are things you used to do together such as: hugging, kissing, and the mattress gymnastics, but are those actions at the heart of it?
I'm assuming you are reading this in order to "get over your ex", "get your ex back" or perhaps somewhere in between. Forgive me if this get a little abstract but I hope I can at least help point you in the right direction.
Every experience you have with someone is reflected onto you and each person you experience something with changes how you see yourself.
How you loved that ex, will never be exactly the same with someone new. This is because your interaction with 'that ex' is unique and thus the reflection of you in that relationship is unique. The love you felt, the things you did are all a unique expression of love that can never be duplicated.
Now, this may sound a bit silly, but what if I told you that your relationship with your ex never really completely ended? Sure the sex, the spending time and maybe the arguing have stopped, but in the end you are left with a story. This love story becomes a part of a greater story of your life. If you think about it, this story we tell ourselves is the story that kinda defines us. Whether your story includes creating vast fortunes, kicking ass and chewing bubble gum or a path fought with adversity and perseverance while skiing up hill, it is a reflection of your character and who you are. You are your greatest hero or your greatest villain. Our story is literally our living creation and we can alter it any way we choose. This is why your story with you ex never really ends and you shouldn't miss them in the sense that you cannot move forward. Your story must go on because you deserve to have a shot at writing your story in the way that you see fit.
I know this stuff might seem pie in the sky, but give it time. If you just experienced a break up, it will be difficult to write your story beyond that event. However, with time and support from family and friends or even our relationship advice support groups, you will be able to see yourself and therefore your story from a different point of view.