Shakespeare's Sonnet 103

Alack! what poverty my Muse brings forth,That having such a scope to show her pride,The argument, all bare, is of more worthThan when it hath my added praise beside!O! blame me not, if I no more can write!Look in your glass, and there appears a faceThat over-goes my blunt invention quite,Dulling my lines, and doing me disgrace.Were it not sinful then, striving to mend,To mar the subject that before was well?For to no other pass my verses tendThan of your graces and your gifts to tell;And more, much more, than in my verse can sit,Your own glass shows you when you look in it.

What poor work my Muse produces! Though she has such a great opportunity to display her talents, the subject on its own is worth more than it is with my praise added!
Do not blame me if I cannot write any more. Look in your mirror: the face that appears there far surpasses my clumsy imagination, making my lines seem dull and putting me to shame. Would it not be wrong, then, to spoil something already fine by trying to improve it?
For my poems have no other purpose than to describe your charms and gifts. And your own mirror, when you look into it, shows you more, much more, than my verse can hold.