You are excited, you're going to sit down with your computer or your pencil and flesh this idea out thoroughly just as soon as you finish what you're doing right now. So you fall back asleep or put your car in gear or finish your conversation or take your receipt, and push that idea into one of the storage units in your brain along with "get up early", "get milk", "call back later", and "balance the check book".
Your day continues. You accomplish everything else on your mental list. You finally have some time to relax so you turn on the TV or open a book or the newspaper or a bag of pretzels. Maybe you remember that you were struck, earlier, by this idea. Maybe you don't. Maybe you actually decide to try working this idea out, so you set aside the book/paper/pretzels, turn off the TV and turn on the computer or find your pencil. You try to remember what the idea was, but maybe you can't. Or maybe you do remember it, so you begin to write it out. You continue for a while, it gets later and later into the night. You're enjoying yourself until suddenly you stop, put down your pencil or take your hands off the keyboard. All of a sudden this little idea that you had doesn't seem so little anymore. The true magnitude and complexity of the thing has just struck you. Sometime between its conception and now the idea has grown into something scary and overwhelming. Your perspective has changed. Now the idea just seems stupid and unattainable. You definitely remember how important it felt to you at the time, but thinking about that now just makes you feel frustrated, and a little angry. You've just wasted countless hours working your hardest at a project that's never going to come to fruition. How could you have been so naive? So you jam your thumb against the computer's power button or throw your (now very short) pencil aside contemptuously, and abandon this bloated, useless, overcomplicated idea to the bowels of your hard-drive or to gather dust on a shelf.
Maybe you think about it for a while afterward. You think about picking it back up sometimes when you catch a glimpse of it out of the corner of your eye, but every time you're almost tempted to do so, you are again reminded of the frustration and disappointment that accompanied it. Eventually you think about it less and less, until maybe you forget about it altogether.
Maybe though, it goes a different way. Maybe one day when enough time has passed that your prior exasperation has dulled, you remember that you had this great idea once. As it turns out, that idea is still interesting and important to you. Your enthusiasm is born anew, and you go in search of the plan that you wrote before. When you finally find it, blow the dust off and begin to read through it, you find that the things you wrote weeks/months/years ago are still very much valid and feasible. Perhaps more-so now than then, and your perspective changes again. This time things will be different. This time you know that this can be done. Having experienced all that former insecurity and frustration has given you a more realistic view of what is required to complete this task.
You start again, but this time you work through the details slowly and deliberately. Allowing yourself time to understand each aspect of the process renews your confidence in the idea. You allot yourself time each day to rework this plan. If you get tired, you take a break. If you get frustrated, you work on something else. You can do this, you're working on your own schedule.
Time passes, and at some point you stop, put down your (last) pencil, and look back at what you've done. The immense complexity of your little idea strikes you again, but this time it's different. This time you're not overwhelmed. The intricacies of the plan are now part of its strength, and you understand all of them. You've laid each step out concisely. You know where to start, where to finish and every step in between. Now this complex idea seems manageable, and at this moment you're reminded of the way you felt at the moment of its inception. That giddy excitement that made you sure that anything was attainable.
The next step is the first step.