The work breakdown structure
Next time you need to tackle a big project, break it down into smaller projects. If the sub-projects are still too big, break them down into sub-sections. Then break those down into action items. You get the idea.
This technique has been around for ages, though in college it was reintroduced to me as the Work Breakdown Structure.
Before you start working on a huge project, you should have a list of prioritized items in your TODO list. Priorities will be defined by your requirements and limitations.
If you have a hard delivery date, you need to think about the duration of each part. If you only have 3 months and one of the parts will take 2 months due to bureaucracy (in life this will always happen) you should start with that on day 1.
Sometimes we face projects that seem impossible to be accomplished on time. This is what you need to know: They are not impossible. You just need to apply the WBS before you start.