Struggling with loss of romantic feelings for nesting partner

My nesting partner (45NB) and I (48F) have been together for almost 20 years; we have all the usual entanglements including a kid. Three years ago we became poly but we'd been talking about it since 2018. Until last year, it had mostly just been my partner who was dating - they got into a long term relationship with a woman, and I had basically decided I wasn't interested in pursuing anything myself. Until I reconnected with an old friend last year (48M) and we hit it off immediately. I'll be the first to admit that there's a high amount of NRE with this person; it's intensified by the fact that it's a LDR and when we see each other it's very all-consuming.

My nesting partner and I have been in couples therapy on and off since becoming poly and have put in a lot of work to make sure our relationship is strong. The thing I'm struggling with is that they seem to need so much reassurance that it's turning me off. And I've started questioning my romantic feelings for them (we stopped having a sexual relationship in 2022, which was one reason my partner was interested in opening our relationship and I supported that). I can't tell if my struggle with romantic feelings (and simple kisses, snuggling, etc) is a result of my new relationship/NRE, the fact that I've been with my nesting partner for so long and things have just naturally faded, their response to my new relationship, or a combination of all three... .

I feel like a terrible person because I get frustrated with their constant need for reassurance and that makes it hard for me to provide it authentically. There is also a part of me that wonders if I'm just bad at maintaining two serious relationships - things were pretty good between us when it was just my partner dating other people. But I'm definitely not willing to give up my new relationship, and I still love my nesting partner very much.

Help??