You thought my heart too far diseased; You wonder when my fancies play To find me gay among the gay, Like one with any trifle pleased.
The shade by which my life was crost, Which makes a desert in the mind, Has made me kindly with my kind, And like to him whose sight is lost;
Whose feet are guided thro' the land, Whose jest among his friends is free, Who takes the children on his knee, And winds their curls about his hand:
He plays with threads, he beats his chair For pastime, dreaming of the sky; His inner day can never die, His night of loss is always there.
Tennyson is now speaking to another person, but we can't be sure who that is. It's definitely someone who has worried about his extreme sadness, so maybe another close friend or a family member.
He's explaining to this person that his grief—which has "crost" his life (meaning, has completely had an effect on every aspect of his being)—has both made him friendly with his fellow man ("kind") and made him like a blind man.
A blind man has to be guided everywhere, and while he can seem happy and take comfort in being sociable with family and friends, he is still totally affected by his blindness.
Or, in this case, Tennyson's grief at losing Arthur—the "night of loss"—will never go away.