I think the one who start feeling it should take the initiative to handle it. He/she should discuss it with the partner. But in reality is it possible. Won't they feel that their partner wud judge them and hold them responsible. So I don't know if it's practical.
Other wise he/she himself should try to mend ways by start caring or spending more time with the partner and try to make new memories which they both wud cherish
First thing to do is...just think about ur present partner think whether u truely love him or her and think if today u can fall for another man or woman then some other day can fall for even better one.. such feelings nvr stops...u will move on on and on...
There is no better way to tell the partner that they are not in love with them anymore.
If both are on the same page, then it may become easier to break up, divorce and move on with life.
But if one partner is dedicated to the relationship, then there can be absolutely no better way since the confession is going to hurt and break them even if actual affair isn't involved.
Most of the relationships at times break because of emotional cheating where partner just develops feelings but doesn't act on them and still he/she finds it impossible to live in the commitment anynore and want freedom from a burdened relationship at all cost. Even in this case the confession will break the partner since the other one isn't feeling those thoughts and their exectations from you are still very much alive.
But for the sake of respect to commitment and human being who is bounded in a relationship because you agreed to it in past, one should atleast come clean before moving on with new partner. It will hurt but atleast some things would be a little better. And relationships shouldn't be making and breaking on the drop of hat.
Often there is a whole step-by-step phase that leads to crumble of marriage, but it always hurts when they break. Even when broken for ego, they hurt people who realize the blunder they did by not even attempting to understand the things and situations.
1st the person has to impartially analyze his own feelings. Has he really fallen out of love. Was he ever truly in love with his partner. Is he really in love with the new person. Is this new love truly worthy of breaking ur marriage esp v difficult if u ve kids. Only if the person is able 2 honestly answer can he really come to a decision. But whatever b the case cheating is a big No. Even if u dont love ur partner she at least deserves respect & dignity as a human being & partner of so many yrs. So its like ripping off the bandage. Talk to her & ask 4 a divorce. Yes it will kill her but better than cheating on ur partner & then she coming to know of tat.
P.S The above points are gender neutral though i ve used the words He & She for simplicity sake. Its true for both guy & girl. Tat's y marriage is a serious commitment & ppl should not marry unless they r ready 2 give their heart, body & soul for this relationship.