why did the computer tell me castling is bad?
I think it is because your queen will have to retreat to the d8 square, and it can not do that if the rook is on that square. On my desktop computer I can find an analysis button in the lower right corner and it's great, it shows which side is ahead according to a strong evaluation machine, and it shows which moves are better or worse. White has gotten control of many squares on the queenside by move 8 so I think developing th queen to d5 wasn't the best plan.
You should have developed your king side pieces and castled king side because white has the a-file open and can easily attack you.