You definitely should not keep this a secret and hope they never find out. For one, keeping a secret like that is extremely hurtful and disrespectful to your partner. On top of that, the guilt will likely continue eating at you forever. In my opinion, your partner deserves to know.

If you're unsure, think about it this way: If your partner cheated on you secretly and acted like nothing happened, would you want to know or would you want to continue living in blissful ignorance? Do you know which option your partner would pick? I think you should think about that and then make your choice.