I'm thinking of what my husband doesn't know
Can you explain what you mean?
I'm cheating on him with my ex.
You shouldn't keep this secret away from your husband. Cheating is not something you can hide forever.
I will keep it a secrete while I figure out what to do with the cheater and the cheated.
You should decide fast. Cheating is a betrayal of trust. If you are unhappy you should break up before you make things worse.