A smaller essay for another module that has an earlier deadline cannot take priority over your dissertation all of the time because the level of work required to complete your dissertation successfully will be significantly greater than the time needed to finalise your smaller essay. So look at the hours available to you each day between now and your dissertation deadline. Factor in the time needed for other activities and other academic assignments or commitments and see what you have left. Now block out the time available to you as dissertation time. By doing this visually, i.e. on a calendar, you are more likely to stick to this timetable of action.