Contrary to popular belief, there is no formal process you need to go through to get a "legal separation". Nothing needs to be filed in Court or even given to your spouse.
In fact, your separation simply occurs the moment that one or both of you lets the other know that you wish to separate with no reasonable chance of getting back together.
Sometimes both of you can have very different recollections of the marriage breakdown and what incident, if any, triggered the separation. This kind of dispute is more common in situations when you both continued living together in the same home, and kept household responsibilities and finances continuing as business as usual.
You should speak to a lawyer to explain your situation. A lawyer experienced in practicing family law can help you better understand the factors involved in determining your separation date. This is important because the issues of child support, spousal support, property division, and divorce, depend on this date.