Have you started a relationship with that new relationship energy and immediately started feeling like this might be the one that clicks? What about having sex for the first time, and it's magic, intimate and feels like the best sex you've ever had, or that you've had in a long time even from your primary partner? How would you go about discussing this?
Let's Talk About Jealousy First
Jealousy is a perfectly normal emotion that rears its head depending on any situation or relationship. This needs to be addressed right away if it does so that things can be resolved rather than left to fester and rot in the relationships that you have and potentially ruin them or cause strife.
Whether it's jealousy in yourself or your partner, talking about it is crucial to having a healthy relationship. Starting with a new partner can be exciting, but you have to keep the others in an open relationship in mind. Rules and boundaries need to be respected and making sure that these are clear before that relationship is started with the new partner is essential.
Getting into a new relationship can often have it's pitfalls, highs, and lows. However, having that N.R.E (new relationship energy) can often mitigate that and send people into a situation where they feel like they have new beginnings or new attractions forming and sometimes neglect the partner at home. This can make the partner at home feel neglected in that this new partner is being pervasive or intrusive into the current relationship and may make them jealous.
Spending time with your current partner as well as that of your new partner and making sure that all needs are met, that you talk about things, check-in often, and make sure that you respect any rules and boundaries that are laid down, is crucial to making sure that the relationship(s) is a success. Your partner may be able to overcome their jealousy eventually by enjoying that you're into this new relationship and experience things through you, but that's not the complete picture.
Figuring out how to love themselves and you loving yourself is a key to making sure that you and they are comfortable with any new relationships that are forming so that there are no thoughts of jealousy or insecurities that crop up along the way. Working on these with a therapist might be a good idea, and having couples counseling might not be a bad idea either.
You can find many couples and individual therapists understanding and sensitive to polyamorous relationships' diversity through quite a few resources online. There are also support groups out there that can help you through the phases of your relationship.
But My Partner And I Have Better Sex
This could be something entirely new to the relationship with that N.R.E that is coming up, or it could be that your new partner is just better at it. It could also be that you and your main partner have just fallen off over the years and become complacent with one another. Women tend to fall off on their libido after 1-4 years, and men tend to stay fairly constant through that timeframe. Throughout marriages, sex waxes and wanes over that time, sometimes it's excellent, and sometimes it's not.
That doesn't mean necessarily that the sex with your primary partner isn't worth having; it just means that you may need to work on things with them a bit to get it back to where it was. Take your experiences with your new partner and what works for you and apply it to your main partner by asking them if you can try new things. Don't neglect them for the new partner.
Don't feel guilty for feeling this way. It's not a bad thing to have better sex with another partner. However, bringing it up to your primary partner in a manner that's accusatory to them about your current state of affairs with them might not be a good idea as it will make them feel attacked and only make the situation worse, and thus the sex, worse.