I'd like to become much stronger mentally, but never forget the time I was weak as well.
この場合、次のような言い方ができますよ。
ーI'd like to become much stronger mentally, but never forget the time I was weak as well.
「精神的にもっと強くなりたいけど、弱かった時も忘れないようにしよう」
to become much stronger「もっと強くなる」
the time I was weak「私が弱かった時」
ご参考まで！
I want to be strong, but I can't forget myself when I used to be mentally weak.
I want to stand firmly on the ground with my own two feet, but I can't forget the time when I was more mentally fragile.
ご質問ありがとうございます。
① "I want to be strong,"＝「私は強くなりたい、」
"but I can't forget myself when I used to be mentally weak."＝「でも精神的に弱かったときの自分を忘れられない。」
② "I want to stand firmly on the ground with my own two feet,"＝「私はしっかりと自分の力で生きたい、」
"but I can't forget the time when I was more mentally fragile."＝「でも、精神的にもっと壊れやすいときだった自分を忘れられない。」
ご参考までに。
"I want to be strong, but I mustn't forget when I was weak."
"I want to be strong, but I mustn't forget when I was weak."
"I want to be ~" 「〜になりたい」
"strong" 「強い」
"I mustn't" = "I must not" 「〜してはいけない」
"forget" 「忘れる」
"when I was weak" 「弱い時」
[must not](https://eikaiwa.dmm.com/uknow/questions/16573/)についてもっと詳しく知りたい方はクリックしてください。
カナダ 
アメリカ合衆国 
オーストラリア 
