I'm married but slowly falling for someone else
Why are you feeling that way even though you are in a relationship?
Because it's not easy to break free from a relationship
It can be common to grow accustomed to your partner over time, and no longer feel as interested in them.  However, you still love your spouse, so it would be heart-breaking for both of you to lose that.  The love you have for another person is probably just a passing infatuation.
I don't care, I love the other person!
If you are adamant about feeling this way then you should tell your husband this. He deserves to know that you love another person. It's not fair to keep cheating on him.