What do you think about me leaving my husband even though I'm still in love with him?
Why would you leave him if you're still in love with him?
I'm having the affair with someone else.
Everyone makes mistakes, and sometimes cheating can be one of those mistakes. Not every partner can forgive cheating, but you should be honest with your husband about your affair and see if the relationship can be repaired. If you leave without being honest, you'll never know if there is a chance to fix things with your husband.
I'm a grown woman so I do as I please. My cheating is my own private business.
Don't you think you should think about your husband's feelings as well?  You're married, and cheating is not a private matter in the situation. You need to be upfront and honest with your husband and see if you want to work things out with him. Continuing the lead him on a lie is going to be detrimental to both your mental health.