First of all I'm sorry to hear about your medical condition.
I don't think you understand that you think this way because you are seriously depressed. Mentally stable people don't think like this.
You can't possibly know why the OP thinks that way. To say that affirming life equals 'mental stability' or 'mental health' or whatever is dogma, not fact. I do not regard psychopathology as science, not unless they can actually prove their contentions and not randomly decide certain symptoms are tied to a mysterous, unobservable 'mental illness'.
In my opinion people do not live because they are happy: they live because it's their nature to do so and their biology and psychology compels them. Optimism is hardwired into our brain but that does not mean that life is swell and everything is worth it in the end. Our mind plays tricks on us. The Polyanna-effect is well researched.
If we were rational creatures we would likely have died out by now. Most of our lives are spent staving off want and deprivation. We are vulnerable to all sorts of accidents, illnesses, losses and the malice of others. Our capacity for suffering is near infinite, our capacity for pleasure limited. Our existence has no real meaning other than what we randomly deem valuable. In the end we die, are forgotten and our lives will have meant exactly nothing.
Given these facts what did we gain by having been born? Goethe phrased this really well in his Faust:
"Since everything that comes into being is worth that it is destroyed it would be better if nothing existed."
Before you claim I'm depressed: philosophical pessimism has a long and rich history and many of the greatest minds professed to it. Unless they were all 'mentally ill' I think it's clear equating optimism with sanity or being 'mentally healthy' and the opposite with 'mental illness' is a modern social construct, not a conviction grounded in reality. During most of history suffering and death were regarded as the normal course of life and people were not obsessed with attaining the mythical state called 'happiness'.