What you “have” to confess is different in Roman Catholicism and Eastern Catholicism. The person asking is Roman Catholic, so I’m giving the Roman Catholic response to her.
A sin is a falling short. It’s being less than God-like. We do that all the time, unfortunately. Little ones like losing patience, eating past when you’re full or spending time online when you’re supposed to be working are all falling short, but they don’t refuse God’s grace. Big ones like adultery, stealing, perjury or relations outside of marriage are a deliberate refusal of God’s grace and require reconciliation before receiving the Eucharist.
1 John 5:16-17 If you see your brother or sister committing what is not a mortal sin, you will ask, and God will give life to such a one - to those whose sin is not mortal. There is sin that is mortal; I do not say you should pray about that. All wrongdoing is sin, but there is sin that is not mortal. (NRSV)
There are criteria they use for deciding when something crosses the line from little to big. Grave matter, full knowledge, free will, deliberate intent.