My heart is smitten - Broken; crushed with grief. We now speak of “a broken heart.” Even death is often caused by such excessive sorrow as to crush and break the heart.
And withered like grass - It is dried up as grass is by drought, or as when it is cut down. It loses its support; and having no strength of its own, it dies.
So that I forget to eat my bread - I am so absorbed in my trials; they so entirely engross my attention, that I think of nothing else, not even of those things which are necessary to the support of life. Grief has the effect of taking away the appetite, but this does not seem to be the idea here. It is that of such a complete absorption in trouble that everything else is forgotten.