How to tell my husband I love someone else
I would go to a therapist first. The person you're in love with is new and exciting and you have a different relationship with them because you're not married to them. If and when you leave your husband, you will begin an entirely different relationship with this other person. It will be completely different than the one you have now - even though it's the same person. Be prepared for it. This is only relevant if the person you're in love with actually wants to be with you in the same way. They could tell you how much they want to be with you, thinking that you'll never really leave your husband. People tend to be more attractive when they're unattainable.
After all of this you may still feel the way you feel. It will be easier to separate from your husband on amicable terms if you work with a therapist first or at least through a mediator.
Please consider if you are tired of the marriage or tired of your husband. You may want to separate first and see how you both feel. I wouldn't recommend telling him you don't love him anymore. This is an incredibly hurtful thing to say to someone who committed their life to your relationship and saying it serves no purpose. Instead tell them that you do not want to be in the marriage anymore. Remember it's not 50/50, it takes two people to be 100% in the relationship for it to work.