The act in and of itself is grounds for breaking up. You're only adding to that probability with each day you allow him to remain in the dark.
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It's unlikely he will never find out about this. I don't think you comprehend the level of betrayal he's going to feel if you withhold this information for months, pretending all the while that everything is fine.
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He will look back on the previous few months and be sick to his stomach that someone he presumably loved and trusted lied to his face (by omission) every day.
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