Could you specify "doesn't work very well"?

Or let me take a guess. You've set up a snippet, for example a template like the following, to simplify things you enter over and over again.

// sketch description: XXX

// globals are defined here...

// setup
void setup() {

}

// draw
void draw() {

}

// custom functions go here...

But if you enter your defined keyword, for example "protemplate" the syntax isn't formatted correctly with the indentation. Afaik, there's an option in TextExpander to set your snippet content to "Formatted Text". This may help.

Also, due to it's cross-platform nature, the Processing IDE is written in Java. Maybe TextExpander doesn't work well in such an environment. From my own experience I know using the IDE sometimes yields untypical behaviour, compared to programs written in Objective-C.

Above speculation aside, I second /u/gliph in saying, that you might have outgrown the Processing IDE. Snippets and Autocompletion are one of the first things people tend to long for when they reach an intermediate level, and more advanced IDEs do offer this. Though I can't say anything about IntelliJ, since I've never used it. Personally, I use Sublime Text, which is an ultra-slick, fast and mighty Text Editor. Setting up Processing to run with it can be a tad fiddly, but there's good instructions out there. The one downside of Sublime is that it's relatively expensive, depending on your current budget. A totally free alternative would be the Eclipse IDE, for which there's a great tutorial on the Processing website.