my mind is overshadowed by a cloud of disappointment
With a subtle touch Mary Shelley emphasizes Walton's similarity to the Victor Frankenstein who, two paragraphs earlier, had been unsure whether he could rid his mind of its "passion," that is, self-interest in its own affairs. The principal cloud overshadowing Walton is the failure of the entire enterprise to which he has been committed for a number of years. The death of Victor has also robbed him of the only deep friendship he has known. On professional and personal planes, then, he is equally bereft.