He was overly confident and ended up making mistakes.
Tips
usage
过于 is more formal and literary than 太 (tài). Both mean 'too/excessively', but 过于 often appears in written Chinese and carries a stronger sense of criticism.
grammar
过于 + adjective/verb: 过于乐观 (overly optimistic), 过于依赖 (overly reliant). It always implies the excess is a problem.
