Could you share some strategies to manage risk and negative performance? What does a professional do when their original investment thesis is proven to be wrong?

If investment thesis is wrong, like you invested into the wrong company then cut loss and move on. Sell the shares when fundamentals change or when your conviction is lost

I am not a professional, and am learning as well.
My strategy to manage risk is to employ DCA on monthly basis to form a base in an ETF portfolio.

In a negative performance environment, the strategy will allow you to purchase more shares and hence balance out the returns when the stock turns uptrend again.

It is a different thing, i think, altogether, if the original investment thesis is proven to be wrong. However I also believe that, as long as proper dd is done, with strong conviction, no investment thesis should stray too far off to the “wrong” side.

Good luck and enjoy the ride!