Alan Kay has agreed to do an AMA today

In "The Power of the Context" (2004) you wrote:

  ...In programming there is a wide-spread 1st order
  theory that one shouldn’t build one’s own tools,
  languages, and especially operating systems. This is
  true—an incredible amount of time and energy has gone
  down these ratholes. On the 2nd hand, if you can build
  your own tools, languages and operating systems, then
  you absolutely should because the leverage that can be
  obtained (and often the time not wasted in trying to
  fix other people’s not quite right tools) can be
  incredible.

However, more often than not, I find the sentiment paralyzing. There's so much that one could probably learn to build themselves, but as things become more and more complex, one has to be able to make a rational tradeoff between spending the time and energy in the rathole, or not. I can't spend all day rebuilding everything I can simply because I can.

My question is: how does one decide when to DIY, and when to use what's already been built?

That is, if I have a problem that requires a library or program, and I don't know of one, I semi-simultaneously try to find a library/program that exists out there (scanning forums, googling around, reading stack overflow, searching github, going to language repositories for the languages I care about, etc) and also in parallel try to formulate in my mind what the ideal solution would look like for my particular problem.

As time goes by, I get closer to finding a good enough library/program and closer to being able to picture what a solution would look like if I wrote it.

At some point I either find what I need (it's good enough or it's perfect) or I get to the point where I understand enough about the solution I'm envisioning that I write it up myself.