There is never a good way to tell your man you love someone else because anything you say means hurt. So, regardless of how the words are couched in gentleness, sensitivity and compassion, it is still going to hurt and be interpreted in a negative way, for three main reasons.
First is the effect on the ego. Masculine ego takes a nosedive when that person feels they are no longer valued anymore. To be rejected for another MAN, especially, brings in the competitive element, and all that will immediately loom large will be sexual: the idea that another penis can pleasure the partner more than his. That is what affects men the most. How HAPPY their partners are going to be when they are with this new person, being loved by this person when they won't be loved by anyone, which makes them feel inadequate in their own capacity to please. This often breeds a lot of resentment and anger.
Second, loving someone else represents total rejection of that partner, what they have to offer, their protectiveness, care, nest building attributes. If they are still in love with the person who is leaving, that is doubly hard to take, because many people cannot imagine or see the reason why their spouse could dare fall for someone else when they have been such 'good' partners. Rejection appears to mean an immediate loss of usefulness, value and low self-perception, and is often very hard to deal with. That is why there is sometimes violence against women associated with a change in partners because some men lack self love and confidence so much, once they are perceived to be rejected by one partner, they interpret that as rejection by every woman on earth. They believe they are finished and turn aggressive as the ego cannot deal with such perceived loss of status and face.
Third, there is immediate uncertainty and fear of the future. Fear of being alone, of not being loved, of not having a 'life' anymore and fear of being unworthy to others. Fear is the dominant feeling when a break-up is threatened, and this fear affects everything else which follows. Fear makes it likely that partners will NOT listen to the reasons for the break, but put their own reasons in to justify how they feel and make themselves more self-righteous and victim-like. Most partners go in denial, refusing to believe that their actions might have had anything to do with the state of the relationship or the decision.
Partners deserve to be told when a new person is on the scene. They should be told factually, without too many reasons why, except that you have fallen out of love, perhaps no longer find them attractive, or just desire a different life - which are all your RIGHT. Emphasise how great they have been as partners, how much you appreciate what has been shared between you and how we all have to accept our emotions. If you start to find reasons relating to your partner why you love someone else, it will merely turn into useless accusations and blame, and escalate the situation negatively. Emphasise that feeling the way you do about your life, and your partner, and your need to grow and develop in a new way, is partly responsible for loving someone else. But it is YOUR desire for something new, something absent from the relationship, which you would like to discover, according to your needs and wants as a living, thinking, aspiring human being.
Once you have told them, then DON'T hang around. Leave that person to deal with it in their own way, instead of being a constant reminder of what they have lost. That is why such disclosures are best left until after you have left the home or when you are actually leaving it. You can always communicate by phone, email or third parties later on. But to be still sharing the same space when you clearly love someone else, especially if that person is truly affected by it, is just rubbing salt into the wound and would create huge resentment and animosity.
There are numerous other ways to leave a partner but the main objective is to do it with sensitivity, empathy and gratitude for their presence in your life.
NOTE: This article is addressed to women because in the UK most fatalities in the home are of women. However, the basic advice would apply to anyone.