Complex sentence: Mandy thought this would be impossible but she gave it her best effort, and to her surprise she was able to learn the lyrics, and this is what made her suddenly realize that she didn't have to give up on singing.
Split into simpler sentences: Mandy thought this would be impossible, but she gave it her best effort. To her surprise she was able to learn the lyrics, and this is what made her suddenly realize that she didn't have to give up on singing.