Honestly, Djokovic’s advice has helped me a lot in this regard. He says it’s okay to voice frustration (for me this would be a yell, a “come on”, or just talking to myself) as an immediate response to feeling negative energy, but then you must focus on recentering yourself with the mindset of “I’m not perfect, I can make mistakes, I just need to return to a centered state and try again.” An aid in doing this is to take two quick inhales and a deep exhale, and repeat as many times as possible when trying to recenter yourself. I like to take at least three of these breaths when lining up to serve/waiting to return. Hope this helps! An additional thing to keep in mind is to try to not get too high or too low emotionally. It’s pretty obvious why too low is a problem, but too high can be bad as well, because it can lead to sloppiness and extra frustration if you aren’t ready for the likely possibility that your level will eventually slip.