>>16 out of C++, Java, and Python, sadly Python is the slowest.
Also, if you find C++ to be an unsustainable mess, you're probably writing messy code. The language itself is pretty straight forward in my opinion, java as well. If you give yourself time to learn to program well in java, you'll find that it's a pretty well defined toolbox, you can also use your experiences from that if you want to switch back to C++.
Long story short, if you find that your code gets messier and less sustainable as the code base grows, look up clean coding principles and clean coding tutorials. Helps a lot, I guarantee you. Also, writing clean code in any language solves many of your performance issues by itself.