Answer

The opening stanza tells of how terrible he feels, as his ‘prime of youth’ is metaphorically compared to ‘a frost of cares’ so the idea of a frost hardening around him, making him cold and taking the life from him and all his cares shows how sad he feels.

He eats a metaphorical ‘dish of pain’ which means he is feeling sorry for himself and everything is painful for him and his ‘crop of corn’ ( metaphorical for all the good things he has done) is turned into a field of weeds and useless things.

All the goodness seems to have gone and all that is left is lifeless and worthless.

(or any one of a number of other relevant examples).