I agree mostly with the other posters here. I think before you go any further with this you might want to go back to that poly friendly therapist. Your family does not seem happy or stable. Please don't seek out unsuspecting women to create a bigger family with without sorting out the issues you two have.
The issues seem to be your husbands lack of self worth and insecurities that make him think its okay to blame you while you pick up the pieces of his miserable break ups; and your willingness to stay in this! Why? Why would you? Please don't say the kids, they are learning everything about how to have relationships from the two of you. Either show them how to take care of themselves and not take on other peoples shit, or show them when its healthier to leave. I don't think you do them any favours by leaving things as is. You don't do anyone any favours really.
I think your question could of been, "what do I do" rather than the one you asked. I think he would do better looking after himself (therapy?) rather than you trying to figure out how to support him. Sure you can say kind words, be their as a shoulder to cry on, but that's about it. Its his journey and he has chosen all of it.
As to feeling old and un-lovable? Really to me there is nothing more attractive than a woman who takes care of herself and doesn't let other people dictate her sense of self worth. Nothing to do with looks, weight, age. Its all inside and what comes out as a result.
You might want to get some ideas about how other poly people have learned some relationship lessons by doing a tag search for "lessons" and "foundations."