Edited
How do you know for sure he's not having an affair? It could be numerous reasons why he stopped having sex with you. It really sounds to me that he may love you but no longer find you sexually appealing obviously. Some people can gain weight and still look nice. Since we don't have a pic of you, we can't judge how you look. The only option is counseling really because none of us on here can MAKE him want to have sex with you. I know our words can be comforting. I know all about emotional eating. You will have to control that yourself. Do not let your eating get out of control or you will have an extremely hard time getting all that weight off and when you do you will have so much excess skin left that you may consider very expensive cosmetic surgery to get back to where you were. Trust me, when you see all that excess droopy skin people have after losing weight, it's enough to make you put down that fork for days. I've seen it and it's no fun. Besides, the more weight you put on the more problems in your marriage you are going to have.
What truly bothers me is that your husband is not being forthcoming with you. He has been with you long enough to talk with you about anything. I'm not buying that tired thing. If he can eat, put on his clothes, take showers than he can muster up one hour to be intimate with you.
Unfortunately, (I know I'm going to get bashed for this) marriage just like any relationship doesn't always stay rosy forever. They start off rosy and then slowly but surely the passion and "I'm so in love" starts to die off a little. Like I said earlier, counseling is your only hope it seems. Don't end up like my relatives did, she ended up over 300 lbs and they slept in a different bed every night for well over 15 yrs and I don't think they had made love for even longer than that. He died of chronic alcoholism and she never married again. She just kept getting bigger, sitting on her bed, smoking cigarettes and talking on the phone about 95% of everyday for years.