It’s great that you and your girlfriend are in love. Often, love means making sacrifices. Do you two love each other enough to make the sacrifice of giving up sex until/unless you get married?
To answer your question directly: In order to make a valid confession, you must have the intention of avoiding sin and the near occasion of it in the future. If you and your girlfriend realize that you made a mistake and want to stop that behavior, then go to confession.
