。
Yoko thought it was impossible to keep living alone.
Literal
Yoko [topic-は] more-than-this [attributive-の] alone live [nominalizer-の] [topic-は] impossible is [quotative-と] thought.
これ以上 ('from this point on / any more than this') is a fixed phrase. 不可能 ('impossible') is a na-adjective with the 不 negative prefix. ~と思う embeds the thought. 独りで ('alone / by oneself') uses 独り (same meaning as 一人 but emphasizing solitude).