Metaphor

"Oh! that my young life were a lasting dream!
My spirit not awakening, till the beam
Of an Eternity should bring the morrow.
Yes! tho' that long dream were of hopeless sorrow,
'Twere better than the cold reality"

The metaphor is his life being pleasant and enjoyable with dream-like parts. He compares that to the cold reality that has parts that cause sadness, mourning, and regrets.