I see no easy way to do what you are going to do.If you want to make it painless then leave him but not because of some other guy,then down the road get together with your new guy.That is as painless as it can be other than not leaving your husband but telling you two need some help w/your marriage!Hope you make the right decision.
It's too late for you to make it as "painless" as possible. You have already been mean and selfish. The damage is done. You did what was easy for YOU and now you're all kind and compassionate. Divorcing him and not flaunting a lover at him first would have been less painful for him.
You cant make this painless its hurtfull!! you need to ask your self if it is worth it before you do it. You need to truly love the other person and in return make sure he loves you or in the end he will leave you for someone else.
Never leave the one you love for the one you like, because the one you like will leave you for the one they love. Realize however way you chose to do this, it is going to be painful. It's a funny thing love, you get good, honest, loving people who would never hurt you - but they remain single, they can't find anyone. Then you get people like you, who are married & have affairs, who cheat, & break hearts then jump right in to the next one! At the end of day it's your happiness that matters, do what makes you happy. I pray your husband gets through this okay.
ah its painful, but its more painful if you wait and hesitate. If you dont love him and you truly truly love someone else then make it quick. It will hurt but not as much hurt as its goina do to you if you stick around and wait.