That is a good question. If you have had time to honestly evaluate your marriage and have come to the conclusion that you ended it too soon without trying hard enough to save it then that's one thing.
But if you, like so many others, are just now facing the fear and uncertainty of being newly single and are starting to think being in a bad marriage is better than being alone you really need to find some help to get over the feelings of loneliness and depression before you do something you'll regret.
If you're positive that getting back with your ex is the best thing for both of you, here are a few simple steps you can follow:
1) Try to talk to him. Not yell or nag, but talk. And listen. Try to honestly and maturely discuss why the relationship ended. What misunderstandings took you so far away from one another.
If the two of you can honestly discuss how you're feeling, without recriminations, than you might actually see that you're not that far apart after all. You have just been coming at it from opposite directions.
2) Take some time to honestly evaluate what part you played in the deterioration of your marriage. No one is ever completely blameless. What was your part and are you willing to change whatever behavior it was that contributed to the breakup?
3) Try to find resources that will help the two of you communicate. So many couples just have a hard time expressing themselves and listening to their partners. If you can find a way to work around this you might have a shot at not only getting back with your ex but also of making your marriage what you'd truly like it to be.