Feeling lonely when you're with someone else is even worse.
ーFeeling lonely when you're alone is bad, but feeling lonely when you're with someone else is even worse.
「一人でいる時に孤独を感じるのも嫌だけど、誰かと一緒にいて孤独を感じるのはもっと辛いわ。」
Feeling lonely when you're with someone else is even worse. で「誰かと一緒にいて孤独を感じるのはもっと辛い」と言えます。
feeling lonely で「孤独を感じる」
ご参考まで!