at the end vs. in the end

 At the end implies a point in time, or it simply refers to the position of something.  It means the time when something ends.

• At the end of his life, he had no regrets.
• We put a period at the end of every sentence.
• I pay my electricity bill at the end of every month.
• We scored a goal at the end of the match.
• In Disney movies, the princess always marries her prince at the end. (Meaning: marriage happens in the final scenes of the movie.)

In the end suggests that something happens after changes or uncertainty. We use in the end to mean a final result or a final resolution of a situation.

• It was a tough game but we won in the end.
• He gave over fifty interviews but got his dream job in the end.
• In the end, parents know best.
• It was a tough match, but we won in the end.
• In Disney movies, the princess always marries her prince in the end. (Meaning: the princess marries her prince after some difficulties and hurdles.