Hi Raspberry Surprise--
I'm sorry you are suffering from poor health. That really sucks, and I think your husband might be accidentally putting too much pressure on you--I'll get to that in a minute.
Interestingly, both my sister and I have been diagnosed with PCOS, and we always thought that it accounted for our ramped up sex drives! We have just that little extra testosterone... it's so hard to predict how hormones will affect each person. Anyway, I'm not a doctor so I'm not going to comment on what might be affecting your sex drive. Different sex drives are totally normal so it's possible that you are just fine and there's nothing wrong with you--it might just be the way you're built, and that's okay!
Okay, about your husband: I think he is making a few rookie mistakes that are going to make this transition a little harder. Is he new to BDSM? Because if he is, he is going to want to explore the scene a little bit and learn a few things before he gets involved, hot and heavy, with dominating just one girl in a long-term love relationship. Very few submissives will get involved in a new Dom who doesn't know what he's doing yet. So what your husband needs to do is go to some BDSM events and get to know people, etc. There's literally a whole sort of community of kinky people that kind of take care of each other in most cities, and he's going to want to plug into that for support, and put off looking for a long-term relationship for awhile.
I feel like he is already accidentally "Domming" you by asking you to pick out the girl he falls in love with. He wants you to be involved, and he doesn't want to hurt you--all good impulses--but I'm not sure if he realizes how much pressure that puts on you. Picking out the girl yourself would mean that you will have to get involved in the BDSM scene too, find out what he likes, get to know people, and then pick out someone he might like. Even then, there's no guarantee he would fall in love with the girl you chose because love doesn't work like that.
If you are truly not interested in BDSM, it might be a whole lot easier on you if he went to his play parties without involving you, then he can see who he likes and let things evolve naturally with any girl he pleases.
Once he starts seeing a girl, he can then bring her to meet you, and you can be friendly and all have dinner, and you can be grateful to her for having rough sex with your husband when you don't want to, and you can all get along naturally. Doesn't that sound... easier?
Also, why would you and your husband stay married at all? It's important to talk about that. If you are allowing him to go outside your marriage to fulfill his sexual needs, which is totally legit, you have to make sure you've found your own niche. Do you still love each other? Do you have children? If he isn't having sex with you much, what other activities do you enjoy together? It will help your jealousy a lot if you two still have something special together. Do you have a TV show you both enjoy where you can sit and watch it and he can snuggle you and rub your back? Perhaps you can go on walks together, which might help your health too a little? Have some way where your relationship can be unique and affectionate, and he has to make sure he is still being your husband in this way.
Or you will just split up.