I did that once. I told my regular confessor that I didn’t have any sins to confess. He said “pride?” OK, so I confessed little things that I did, like getting annoyed at someone who cut me off in the car. Usually I don’t have anything big to confess so I confess little things that I do, like maybe engaging in a bit of gossip, or skipping on of my prayer times (I didn’t think this was really sinful but it could lead to a lessening of my prayer life)
We can declare ourselves sinless all we want. I work with protestants who tell me that they are sinless because they have been forgiven for sins they haven’t even committed yet. (I didn’t believe this before I became Catholic.) As a Catholic, what I believe is that there is only one way to know that I am sinless . . . when Jesus the Christ Himself speaks the words of absolution through a priest acting in persona Christi.