In the examples given, using "za" places much more focus on you. - "Tesko mi je" all by itself like that means "I'm having a hard time", a general statement focusing on being hard. - "Tesko je za mene" means "this is hard for me", focus being on you. It could be a contrast like "za tebe je lako ali za mene je to tesko" - "To mi je bilo veliko iznenadjenje / bilo je to veliko iznenadjenje za mene" again, the difference shifts the focus from a surprise to a surprise for you. Maybe it wasn't a surprise for others, even though that's possible in the first sentence as well. It's a slight difference. "Bilo mi je lepo" sounds right but "Bilo je lepo za mene" is not that common. You could say "Meni je bilo lepo" if you want to underline that maybe it wasn't for the majority or for the others, or as a contrast to someone who said the opposite before you. Not an in depth answer but I hope it will help once you combine it with all the others that you'll get. Good job as always!