Sometimes Christians fixate on past sins. But we cannot experience the joy of salvation while obsessing on past sins. Christ's blood covers sins repented of.
Sin creates estrangement from God, causing us to fail in everything we attempt. Sin always produces separation; it never heals, but causes death.
Righteousness consists of applying the Law's letter and/or intent. Sin constitutes a failure of living up to the standards of what God defines as right.
Simply watching out for the so-called "big sins" suggests that we are not genuinely interested in conforming to God—just in not crossing a major red line.