I tend to work on small projects, and because of the way I'm wired (i.e. because I haven't rewired myself out of it), when I'm working on something larger, I can't really keep many pieces of it in my head at once. When I try to, bits which aren't drawing my attention right now will fall out of my brain, and sometimes those bits were important. So I have to focus on small pieces at a time[...]

Ye hairy gods, do NOT attempt to rewire yourself out of this. :-)

I don't know of any programmers who are able to productively hold very large chunks of implementation in their head at once. Writing in small pieces with frequent result checking is nearly always a good idea even if your result checking isn't regressive or automated. You should be happy you started out with this habit instead of having to force yourself into it.