I understand your stance. Everyone has mental struggles. Some of us deal with it. Some of us do something about it. (see a therapist). Some don't, even resulting in suicide.
However, you simply misunderstand the severity of mental illness. This isnt just a, "ugh my job sucks, im stressed." Or "My kids are driving me insane."
You may be fortunate that your personal mental illness has been relatively manageable, but that doesn't mean the magnitude is universally equal. Theres a reason why psychiatrists exist. Theres a reason why medicine is increasing its focus on mental illness. Its not a simple "toughen up kid."
Id even argue that Mika or any professional athlete, a 0.001% of the population is more susceptible to mental pressures than the majority of us working our 9-5 jobs at home as a regular civilian. As an athlete all eyes are on you. You get paid to perform and when you don't, the fanbase turns on you.
Having said all that, Mika has not been good and needs a shakeup. There is no denying that. But its understandable (and okay) to be both frustrated as a fan and have empathy for a player who is struggling.