Analysis of Sonnet 21: So is it not with me as with that muse

So is it not with me as with that muse,
Stirred by a painted beauty to his verse,
Who heaven it self for ornament doth use
And every fair with his fair doth rehearse,
Making a couplement of proud compare
With sun and moon, with earth and sea's rich gems,
With April's first-born flowers, and all things rare
That heaven's air in this huge rondure hems.
O, let me, true in love, but truly write,
And then, believe me, my love is as fair
As any mother's child, though not so bright
As those gold candles fixed in heaven's air.
    Let them say more that like of hearsay well;
    I will not praise that purpose not to sell.