I never thought it would be this trying to have a long-distance relationship with someone until I experienced it for myself.
この場合、次のような言い方ができますよ。
ーI never thought it would be this trying to have a long-distance relationship with someone until I experienced it for myself.
「自分で経験するまで、遠距離恋愛がこんなに辛いと思わなかった」
trying「苦しい・辛い」
to have a long-distance relationship「遠距離恋愛をする」
ご参考まで!