Answer:
He fears the remaining portion of his life will be useless.
Explanation:
Sonnet 19 by John Milton is a lament on the loss of his vision. At the age of 42, Milton lost his vision.
He laments the fact that before he crossed half of his life he got blind and now he will not be able to use his God-given gift or talent for his purpose. As for a poet becoming blind is desperate because now he will have to be dependent on someone else for his works.
This identification of loss of vision by the poet emphasizes the despair that now the remaining life of the poet will be spent in darkness and as useless.
