You must be diligent in completing your task. Every project I have met has expanded to fill the time alloted. If I have 24 hours to complete a task, it is finished in 23.7 hours. If I have two weeks to complete the very same task, I will have it ready the evening of Day 13. Diligence ensures that you work on the project consistently, not waiting until the very last moment.
Balance follows closely on the heels of diligence.
You must order your days so that they are not all work, or all play. If the two week project were completed by the second day, that would be great for the remaining twelve days. All tasks are manageable when you can see easily completed, individual tasks. Decide which segment needs to be taken care of first, second, and so on. Depending on how your particular project divides up, you may choose to complete just one or two segments a day.