Should I be funny with my brain?

As you've discovered, rational and proactive self-talk is amazingly effective for putting a stop to the irrational negative thoughts that are often behind symptoms! That right there is evidence that you're on the right track. The only adjustment you need to make is to let go of expectations.

Keep practicing this technique, while being mindful that you don't put a bunch of pressure behind it. Celebrate and give yourself credit for the fact that it works for a silly new symptom. Remind your brain that "you know, that leg burning is also not necessary" and let it go at that - with zero expectations.

Some day your brain will let it go - but there is no way to know when that day will come. The fewer expectations you have, the more likely it is that the change will happen while you're not even paying attention.

The bottom line is that Expectations and Pressure add to self-imposed negativity, which is how you end up using unhelpful and self-punishing words like "fail". Which is totally unnecessary!