The problem is that I cannot prove that it's mine, unless it's copyrighted
But it is copyrighted. That's what I said in my first reply. You automatically own the copyright.
But I take your point about protecting the source code. Ultimately, if you distribute source code, you leave yourself open to someone ripping it off. You can take measures to show that it is your copyright, but that won't stop someone stealing it - or, more likely, stealing bits of it and using it in their own applications. That's always going to be a danger with source code.
That said, the danger is rarely as bad as people fear. Remember, for someone to actually profit from your work in this way, they not only need the source code, they also need the time, energy and skills to make use of it in their own product, and they would still need to put the same effort into marketing, support, and all the other things that a successful software vendor needs to do. Is your application really so valuable that people would be willing to do that, and also risk the legal consequences? (I'm not denigrating your product. I'm simply questioning whether it would really be worth the time and effort to rip it off.)
So, my advice would be to display prominent copyright notices in your source code (and also your promotional material), but not to get over-anxious about it. If you genuinely believe that you are open to theft of the source code, then, as a last resort, switch to using a compiled language like C#.