Even when I take medicine, the pain doesn't go away which makes me really worried the pain will never stop.
Painkillers don't help with the pain which makes me wonder if I'll have to live with this for the rest of my life.
この場合、次のように言えるかと思います。
ーEven when I take medicine, the pain doesn't go away which makes me really worried the pain will never stop.
「薬を飲んでも痛みが治らないから、一生そうなんじゃないかとすごく不安になる」
ーPainkillers don't help with the pain which makes me wonder if I'll have to live with this for the rest of my life.
「痛み止めが痛みに効かないから、残りの人生、痛みと生きていかなければいけないのかなあ」
ご参考まで！
カナダ 
