When I notice myself doing this, I try to break the problem down into a sequence of high-level subtaks, usually in the form of a to-do list. Emotionally/metaphorically, instead of having to cross the obstacle in one giant leap, I can climb a ladder over it, one step at a time. (If the subtasks continue to be intimidating, I just apply this solution recursively, making lists of subsubtasks.)

I think the most important aspect of this, for me anyway, is being able to dump most of what you're working on out of your working memory, trusting yourself that it's organized on paper, so that you can free up more brain space to do each of the sub-parts.