You will be the sun and moon to this person. You will be their hero. But with that privilege comes a great responsibility. They will keep you on the pedestal so long as you keep up with the things that they need from you. You once thought that this pedestal was a good place to be and knew that you will one day get the appreciation that you deserve for selflessly giving up of yourself. You smile at the insults because you know that he really doesn't mean what he says. He is just frustrated with stress. You are thrown off balance by his gifts and finally realize that most of those gifts are just things that he liked, not necessarily things that you liked, such as clothes or perfumes that he wanted you to wear. He buys you gifts that he wants, like maybe a kayak or two. You begin to wonder where the benefit is for you in staying in this relationship and you begin to pull away.
As you pull away, he begins to devalue you and drain you. Often he disappears or gives you the silent treatment. He took you off the pedestal and you don't know what happened to make him so evil towards you. The answer to that is that you are no longer giving him the supply of admiration that he wants. You are no longer building him up as you once did earlier in the relationship.
You find that he has another supply of admiration and he goes to her for that support. She tells him how handsome he is and how smart and clever. She tells him how she understands and needs him and only she can fill this void he has. She gives him the admiration that he so craves and tells him how he is doing the right thing. He would eat up the attention that she is giving him because he is selfish. He needs that more than anything else in the world.
When she begins to get tired of filling this need, she may become a little more distant and start telling him that he needs to get help or something that he considers as being less than perfect. He cannot handle being less than perfect. He needs someone to affirm that he is right and he is great. He comes back to his old source of validation and praise. In his mind, he has punished her enough and she will be happy to have him back by her side and will fulfill his need for praise.
Don't do it. Don't take him back. The cycle will not change. He will not change. As hard as it is to do, stay with NO CONTACT. Be thankful for the other source of validation that he has. It is this other source that will give you the time to heal. Don't let him come back to you. He may try. But eventually he will return to the other supply or find another. You are worth so much more than that. You are strong and amazing and your light can shine for others.