Complex sentence: He was born into an upper class, but not wealthy, Roman Catholic family and moved to London in 1821 where he exhibited his portraits at the Royal Academy, but not selling enough he changed to lithography for which he was to become famous.
Split into simpler sentences: He was born into an upper class, but not wealthy, Roman Catholic family. He had exhibited his portraits at the Royal Academy, but not selling enough, he changed to lithography for which he was to become famous.