Analysis of Sonnet 50: How heavy do I journey on the way

How heavy do I journey on the way,
When what I seek, my weary travel's end,
Doth teach that case and that repose to say,
"Thus far the miles are measured from thy friend!"
The beast that bears me, tired with my woe,
Plods dully on, to bear that weight in me,
As if by some instinct the wretch did know
His rider loved not speed being made from thee.
The bloody spur cannot provoke him on
That sometimes anger thrusts into his hide,
Which heavily he answers with a groan,
More sharp to me than spurring to his side;
    For that same groan doth put this in my mind:
    My grief lies onward and my joy behind.