Talk to your spouse about how you are feeling in the relationship and ask for her feelings as well. She may be feeling the same way as you are, but is afraid to voice it. Make sure to listen to her and try not to force your opinion that divorce is the choice to make. She may come to that conclusion on her own once she has a chance to talk through her feelings.
Reassure him that he will be as active as he wishes in the life of your children, if you have any. Many times a spouse will stay in a marriage because he wants to do "what is best for the children." If you are arguing frequently or visibly upset much of the time, that is equally as unhealthy for the children. Let him know that he is not going to drop out of the lives of his children.
Consult a mediator. If there are other issues that need to be resolved before your spouse will grant a divorce, a mediator can help to work through those problems with a clear head. Finances, property and other items can sometimes cause the spouse to not be certain about where she stands after the divorce. A mediator can help her see the big picture.