NEW ANSWER NEEDED

Hello! My name is Michael and I am seeking your advice and knowledge in finance, specifically, modern portfolio theory. I heard a phrase once that was attributed to Don Quixote: It is the part of a wise man not to venture all his eggs in one basket (Miguel de Cervantes). I did not follow this advice and invested $10,000 in First Solar (FSRL) 11 years ago (and kept investing in this company for that period). Solar energy was (and I believe it is) the future. Who can disagree with that? However, First Solars 11-year return was -60%! Yes, I have lost money because I invested all my money into just one company, which I thought would provide me with the big return I wished for. What did I do wrong? I researched and analyzed First Solar and its prospects were outstanding 11 years ago and yet its average return has been negative since then. Is there anything I can do now? What would you recommend I do? Answer Michaels questions and use modern portfolio theory to explain what went wrong in Michaels quest for a big return.