Complex sentence: I've contributed to this significantly over the last few years, since I have an ambition to expereince an opera in most of the great European houses, having attended perfrmances from 1959 onwards at the Royal Opera House, Covent Garden in London.
Split into simpler sentences: I've contributed to this significantly over the last few years, since I have an ambition to see opera performances in the majority of the great European houses. I started by attended performances from 1958 onwards at the Royal Opera House, Covent Garden in London.