Once you remember an unconfessed past mortal sin, the obligation is to mention it the next time you go to Confession, but there's no obligation to go to Confession to remedy the situation ASAP.
You are not in a state of mortal sin due to a forgotten past sin. I would heed your Confessor's instructions not to go more than once a week, unless you actually commit a NEW mortal sin (which is what he clearly had in mind).