I'm definitely not a relationship expert an I'm only 18, but I definitely think I have some good advice.
A man, especially your husband, should love, support, and care for you whether or not you are fat, thin, cranky, happy, or upset. A real man in love loves you even in the times that it's not ideal for him. I'm a college aged girl, and I can NEVER imagine myself sleeping with a man your age. I'm just assuming that he's your age. There is definitely something sick going on there, and his "insecurities" are absolutely no excuse for his behavior. Faking break ups should be the last straw.
In the end, it's about your happiness. I know you are insecure about the saggy skin, but a man that truly loves you will not care. Your husband does not love you, or at least doesn't show it. My mother lost over 200 pounds with a gastric sleeve and is worried that a man will never find her attractive because of her skin. But like I said, a man that loves you won't give a crap. The job of a husband and wife are to make each other better. That's what you vow to do. Your husband is not making you better and is only making you unhappy. I personally think that there is more to this than "insecurities". He's a flat out cheater, and I think you are giving him too much credit. It would be one thing if he ended the relationship and was remorseful, but the fake break ups etc. are only showing that he does not care. He is just going to get better at hiding it, and even if he breaks up with this one, he is going to find another.
In lay man's terms, dump him. I know it will be devastating, but he is doing absolutely nothing for you. There is no amount of insecurities that would constitute his behavior. You are a damn good wife, and you deserve a damn good husband, which he is not. By sitting around, you are just permitting this continutal cheating behavior. He may seem remorseful, but he was good enough at hiding this relationship for a while, I think he can be good enough at faking remorse. If he was remorseful, he would have ended that relationship, which he has not.
Please, for your own health, you need to dump him. I know it's your husband and you love him, but he obviously is not treating you with the respect you deserve. You deserve someone so much better who will love you enough not to lie to you.
I honestly wish you the best of luck. Seriously, I do not envy your situation. But honestly, it comes down to what's better for you, yourself, and he is not the best thing for you.
Elizabeth