Is it considered cheating for a wife not to inform her husband about a previous relationship that involved touching the external genitalia, from which she has repented?

What you mentioned does not constitute deceit towards your husband or a sin upon you for concealing this matter from him. Rather, you must conceal your own faults, based on the words of the Messenger of Allah, peace and blessings be upon him: "O people, it is time for you to cease violating Allah's boundaries. Whoever commits any of these abominations, let him conceal himself with Allah's concealment. For whoever reveals his misdeeds to us, we will apply Allah's law upon him."

As long as you have repented with a sincere repentance, there is no reason for your fear and distress, for repentance erases what came before it. So, turn away from these whisperings and thank Allah.