Kant does explicitly address the question of suffering and suicide, rejecting such conditional applications of morality, but unfortunately I have no books presently at hand, so can't provide a reference or details of his reasoning.
Even without the details, we can appeal to one basic aspect of Kant's transcendental and "deontic" morality. The case for "ending suffering" is clearly a personal and "consequentialist" approach to moral acts. The act is based not in the universal structure of rational being itself, but in the perceived consequences.
Kant argues that such justifications presume we can predict the outcome, which we cannot. The limits of "pure reason" deny us this very knowledge. Here is precisely the slippery slope of secular, utilitarian ethics he was keen to avoid. In the case of personal suffering, two considerations intercede.
First, we cannot know the outcome for others of our willed death... or of our reluctantly continued life. Second, we cannot even know the outcome for ourselves, since we must accept, as Kant did, both the limits of our knowledge and the a priori assumption of the immortality of the soul.
Moreover, the structure of the "categorical" imperative as the "willing of a universal law" is meant to transcend personal, contingent circumstances, especially those guided by mere physical pleasure or suffering. By responding to such contingent determinants, we slip from the realm of "rational freedom" into the natural realm of mere physical reaction amid brute cause and effect.
To justify suicide, we would have to assume that somehow bodily suffering is itself inherent to "rational free" existence, leading "rationally and freely" to self-termination, which obviously cannot be universally compatible with such "rational being." (It is worth noting that without Kant's Christian assumptions, existentialists might hold similar views, yet extend human freedom to the uniquely human act of suicide.)
However, I have not addressed the vexed question of how exactly one is to judge various specific "actual" conditions in applying the categorical imperative, which has been criticized on precisely such grounds as an "empty formalism." I hope others can provide more detail, and the suicide citation. While most people would not accept Kant's pseudo-Lutheran strictures today, there is a great deal of wisdom in his concerns about the plasticity of "consequentialist" ethics.