Still, thou art blest, compar'd wi' me! The present only toucheth thee: But Och! I backward cast my e'e, On prospects drear! An' forward tho' I canna see, I guess an' fear!
In spite of all the mouse's little mouse problems, though, the speaker says that she's "blest" compared with him, because she lives only in the present moment.
But alas! says the speaker. ("Och!" is a common Scottish exclamation ranging from annoyance to despair.)
The speaker, unlike the mouse, casts his eye backward to look at the past, when his outlook was dreary.
(Note that Burns uses yet another weird contraction here—"e'e" instead of "eye.")
And the speaker, unlike the mouse, also looks forward, to the future—although he cannot ("canna") see it, he guesses and fears what the future might hold.