I'm not sure I would see this as a threat to your primary status. His love with you is unique. You will always share something in a way with him that is special to you and him. He enjoys the uniqueness that is you, otherwise he wouldn't want you still in his life. He just wants to appreciate the uniqueness in others as well.
I would examine the root of your jealousy and fear. Are you scared of losing him, losing precious time with him, or what? Once you figure out that, then it will be easier to work on it with him directly.
Keep in mind change is a natural part of relationships. It may be scary, but learning to deal with them in a healthy manner is important to determining whether a relationship lasts for the long term or not. Make sure you understand exactly what is making your emotions so volatile, and find out what possible solutions are. This should be done together with your partner, so his feelings are considered in the solution-finding process.
You also may need a small break (maybe a week) to just indulge yourself away from the worry of it all. It is easier to tackle these problems when your mind is fresh than when it is drained. I made some bad decisions when being super drained, so I highly recommend prioritizing getting yourself in a better state of mind. Meditate, get some sleep, whatever you need to do to relax.
I hope this helps some. A new person is always a crazy-scary thing at first, if you're not used to dealing with the insecurity of it.
Me: K, female, 27. Married to Y for over 4 yrs (male, monogamous, 33). Opened relationship to E (male, monogamous, 27) in a relationship vee.