I had had bad posture, but since I came here, I have had good
posture.와
It had been very hard for me to write diary until I came here, but
I have now been good at doing that.
두 문장에 시제 오류가 있으면 잡아주세요.

I had had bad posture, but since I came here, I have had good
posture.와
It had been very hard for me to write diary until I came here, but
I have now been good at doing that.
두 문장에 시제 오류가 있으면 잡아주세요.

문법과 가까운 얘기만 쫌 해보면
위: 중문에서는 시간차 있어도 같이 과거로 had 하는 게 더 자연스러워요. here 뒤 콤마는 없는 게 나음(독립된 부사절+주절인 경우와 다름)
아래: 전반부 시제는 그렇다치더라도 후반부 현재완료와 now 같이 쓴 건 어쨌든 한쪽을 고쳐야.....
관사도 복수도 없이 diary?? ..... keep a diary 하세요.