It is very difficult indeed. If the US stays out then the Entente is crippled and can probably be defeated in 1918. Germany is suffering too and needs to end the war, so the idea that they just sit there in 1918 doing nothing is really quite improbable. There's no need to ask for an Armistice if the enemy isn't advancing. It wasn't coincidence that the German request for an Armistice came after the Hundred Days, not beforehand.
Hang on, here's a thought. No USW in 1917, no US entry, Entente finances and industrial production crippled.
1918 - Germany redeploys west but, misjudging the Entente's strength, either does not attack or makes a half-arsed effort that the Entente manages to repel, but refuses to ask for an Armistice. Germany decides that the war cannot be won on land and so the USW lobby, saying it can end the war in six months with minimal loses, gets the go-ahead, resulting in US entry and reinvigoration of the Entente.
That probably leads to an Entente... well, American... victory in 1919. Alternatively, the USW lobby loses, and in 1919 Germany realises that she wasted 1918 and does a proper attack that succeeds and forces an Armistice request.