Sometimes, you need to wait till the end to start.
Often when I write, I write a clear summation of the idea at the end. Then – at least half the time – I cut and paste it to the very beginning. Part is clarity: I like to start with the clearest form of my idea, and then elaborate. Another part is my own mental evolution: I don’t always know what my main idea is until I’ve written things down; so the main idea comes at the end, when I understand it for myself. Moving it to the beginning is a bit of logical magic – it seems like the first idea, but it’s really the last. By moving it, I change the impact of the entire piece.
The same is true for musical introductions. I don’t always know how to start a piece until I’ve written it through to the end. Some musical idea that I use to conclude a piece might make sense at the beginning, too. Leave some blank bars to fill in later, or just insert them after the fact. Trust that the process of writing the piece will help you understand what it means, and most importantly how it will begin.