Separation is used in three different contexts in Maryland.
(1) Physical separation is grounds for divorce but it has to be under separate roofs. There are other grounds, such as cruelty or adultery, which can be brought even if you are living together.
(2) A Limited Divorce, which is a legal proceeding, is called a Legal Separation in some states. The Ricketts case said you could maintain a limited divorce action based on constructive desertion even if you are living together.
There is always a possibility. In determining alimony, duration of the marriage is one of several factors the judge must consider, as well as health and income earning abilities of the parties. There are benefits and costs to hiring a lawyer and it is a decision each individual must make.
Annulments are granted by the court only in certain rare cases. The legal effect is to void a marriage from the very beginning--as if the parties had never married. The reason must be something that goes to the heart of the marriage, like certain types of fraud, incapacity due to age or condition, or inability to marry because you are married to someone else. There is no time limit. But you will probably find it easier to get a divorce.