I had studied in the US for six months. I had made many friends there.
I had been to England for six months. It had been my first time there.
Past perfect is for when you are talking about the past (using simple past tense) and then you talk about something that happened BEFORE that time.
For example,
I went to Tokyo last Sunday. (Before that) I had been sick on Friday, but fortunately my sickness had got better before I left.
If you are describing the time when you lived (or ‘were living’) overseas, you can use past perfect (had thought/been/done) when you talk about something you HAD THOUGHT or HAD DONE BEFORE you went overseas.
I studied in the US for six months. (Before I went) I had thought it was going to be very difficult to make friends, but in fact I made many friends there.
I went to England for six months. It was not my first time overseas. (Before that) I had been to Canada and Australia.
