I slept with someone who is in a relationship.
Does the person they are in a relationship with know?
No, we're keeping it a secret from them.
Anybody who is in a relationship is off limits, you shouldn't sleep with them. You could ruin people's relationships by doing this. Only pursue people who are single, and interested in you.
I think I can convince someone who is in a relationship to want to date me. They will understand.
They might understand, but they might not.  The person you slept with seems to think that it was a good idea to keep this secret, so you should respect that person's wishes.  While it wasn't good to sleep with this person in the first place, at least you can avoid causing them trouble.