Relationships can be hard to rebuild after having separated. The reason for the separation in the first place is usually to see if you can live without the other. Having this time to yourself to think over what is going wrong with your relationship should help give some insight. But, many times a separation is seen as the beginning of the end.
Every separation does not have to go this route. Relationships can turn out stronger than before after a separated couple had a chance to get some insight into what is going on. You should make the purpose of your separation to fix your problems and make sure they do not return.
Many times people intend to fix their relationship problems but forget to fix the real reason the problem was there. They might get back together but one of them fails to maintain what they need to do to keep it fixed. Next thing you know they are separated again, maybe for good. This would be called relationship maintenance.
While separated, both should be trying to find out what the real reasons were for you to separate. You must get to the root of the problem to fix it for good. And do not just fix a symptom of it, you must fix the actual reason. If you do not do this the relationship can only fail.
Try to look at your issues as an outsider to see if you can leave your emotions out of it. Being emotional over it can mislead your decision making process. Once the problem is identified, you and your partner need to work it out together. Relationships have two people involved and you can fix the problem together.