Automated QA for a single freelance developer?

Automated QA for a single freelance developer?

I have been developing an application in my free time using Qt. As the
size of code is increasing I am finding it difficult to contain new bugs
for older code. I have been testing my application manually. Since the
target is an exe I cannot test it automated with C++ tests without
injecting some extra code into my application.
So my question is, what is the best QA technique for a GUI application if
you are a single developer & wont be earning money from the project as it
will be released for free? Thank You.
EDIT: I would like to have a set of simple tests, each testing for
specific functionalities of my software. I would like them to run
automatically one after another. Finally they should create a report of
which tests failed. This can possibly be done by creating new functions in
the same classes + adding some checks in existing functions I want to test
& then create a new class which will have all the tests. So I wanted to
know whether is this the best way or is there a better alternative?
Because everytime I will build a release target, I will be
commenting/deleting this QA code, which may create some bugs for that
build.
Currently I am not worried about documentation & comments as I have
maintained that from the beginning. It is only about source code QA.