あの時彼女に会いに行っていたらって英語でなんて言うの？

あの – that
時 –time
彼女 – her
会いに – to see
行っていたら - If I had gone

ちなみに、” I could have told her what had happened to me.” は「私は彼女に何が起こったのかを伝えることができた。」

この場合は「If I had gone to see her at that time, I could have told her what had happened to me」になります。