I have tried to present my views regarding these two seemingly related phenomena.
(a) Thinking that Life is not worth living
When one "thinks" that life is not worth living, that is a product of their mind, of their thinking. It is driven by external circumstances that lead them to this conclusion, regardless of the fact whether or not life is, actually, worth living. Most of the time, this external factor is pain, although I cannot think of any other factors presently. We say that life is not worth living because "something". This "something" is an external factor, and not a property of ours.
(b) Being unable to bear the pain of existence
When someone is unable to bear the pain of existence, that is a result of their innate nature, or their own "incapability". This is not due to any conclusion they arrive at about the futility of existence. This "incapability" to bear the pain of existence comes from one's own judgement about oneself. I decide that I am unable to bear the pain of existence because pain makes me uncomfortable, and the degree to which we can tolerate comfort is subjective. Thus, is it more "real" in a way.
However, if one says that their life is unbearably painful, thus it is not worth living, they would be assigning the "lack of pain" to be the only measure by which we can decide the worthiness of life, which would be wrong. Pain is a phenomenon that makes life hard, but we cannot say that its lack thereof would give life meaning. This is because the "lack of pain" is not a yardstick by which we can judge the worthiness and meaningfulness of life, due to the fact that life cannot be assigned any objective meaning.
Please correct any fallacies in my argument, I am a novice in philosophy.