Cut your son loose. At 23, you need to not enable him. He needs to learn to deal with his mental problems for himself.
Now, before you think I'm heartless, I have a very severe case of bipolar disorder myself. So, I have nothing but his best interest at heart by saying this. He needs to be forced to deal with his problems. If he's got a crutch, he'll use it.
As for the weight, you need to put yourself first. You just do. Your children are grown, oh, and so is your husband, who must alsoi figure out how to deal with his bipolar for himself.
I'm not sure why you think you're responsible for all these other grown adults? Is is a control thing? Learned co-dependence?? You're absolutely NOT.
Advice on separating? If you love each other then no. Get some IC for yourself to figure out why you feel so responsible for others and cut it out. With grown children "work, family and being tired" and no time to exercise and having to cook two meals should only be work and possibly tired. How much do you work? If you work less than 60 hours a week then you should be able to make time to exercise, and the more you exercise and eat healthy the less tired you will be.
Cook for yourself and make your husband responsible for himself. You can tell him what you're going to have and offer to make it for him too, if you'd like, but you don't have to.
Finally, there is a lot to be said for pre-packaged diet plans. I've done Medifast with a TON of success. I've heard good things about other plans as well, but it certainly cuts out cooking, and makes the initial weight loss phase a no-brainer. You have to learn how to maintain later, but you have the motivation of having/keeping the weight off at that point.
And, gently, you really do deserve better than a cheating bipolar man who would suggest you are anything other than beautiful ever, especially when you're dealing with so much. 40 pounds isn't a miserable amount. I've had 5 kids so I've gained more than that (and lost it) multiple times. If you're consistent you can lose 40 pounds in 5 months or less.