translation of sonnet

Hi, I'm trying to translate the sonnet "When I have fears" by John Keats. But I feel the rhythmic condition of sonnets is quite hard to be satisfied. For example the first sentence: "when I have fears that I may cease to be", my translation is "ya Rucin au hautan nefë", but it doesn't meet the condition of "iambic pentameter". Are there any suggestions or is there any trying of translating some famous sonnets into Quenya before?

Sonnets in strict iambic pentameter don't lend themselves easily to a language like Quenya, whose prosody is quite different from English.

Your sentence reads "that I feel fear away I cease to breathe air." I'd say something like Yá umbe nin i cé telyauvan náve(nya)... "When I fear (lit. 'when there is an umbe upon me') that I may finish (my) being..."