Both words describe strong belief, but they behave differently at the screen. Conviction says this is my best idea and here is what would change my mind. Certainty says there is no case against it, which quietly removes the stop.
Certainty is usually manufactured by the position rather than by the analysis, and it grows with size, time held, and how publicly you have committed to the view. It is the emotional signature of motivated-reasoning.
Keep a numeric expression of conviction and a written invalidation for every trade. If you cannot write the invalidation, you do not have conviction, you have certainty, and it should be traded smaller rather than larger.
Related: confidence-calibration, overconfidence, belief-perseverance, motivated-reasoning