I cannot recall if you said you are interested in a sexual or romantic relationship with a woman. Forgive me if you've said already. Are you bi or queer?
If you are not, or just a bit attracted to women, then why in the world would you try and date a woman with your partner?
Also if you are truly uninterested in looking for other partners either just for you or as a couple then don't date anyone right now. It would be a disservice to all involved. Just because your partner wants other partners doesn't mean you need to date too. You could be a poly person who is not dating or looking right now. Lots of people do that. Or you can be a mono person in a relationship with a poly partner.
Adding another person to your dynamic with your partner will not result in less jealousy or unhappiness for you. Dating the same person is no magic bullet against jealousy or other painful emotions. You will have to deal with jealousy and similar problems anyway.
Do you feel like you've made progress in understanding the causes of your jealousy? Does your partner? Do you feel like you have some tools to help you understand? You can certainly ask for more time from your partner, especially if you are working on jealousy issues. There is no timeline where at the end you are suddenly totally ok with poly. But that should not be an indefinite time where you and he do not address your concerns and his.
Finally, maybe you are not meant to be poly. There is nothing wrong with that. There is also no timeline to figure that out either but it may be you two are not suited to each other.