Tl;dr I'm experiencing my partner feeling jealous of my other relationship for the first time ever and it sucks :(

Long version: I've been with my partner, let's call them Fern, for about a decade. We were both practicing polyamory when we met - they've never done anything else, but I was pretty new to it at the time - Fern was my first "serious" poly partner. We were both seeing other people at the time and has continued to do so throughout our entire relationship.

We have a really high level of entaglement at the moment - we live together, coparent three kids, share financial responsibilities, etc, so we're by necessity involved in each other's other relationships in the sense that we can't really make plans independently. If Fern is off seeing someone else I definitely know about it because I'm either commited to staying home with the kids or arranging childcare. And if something's going on in their relationship I know about it because we spend a lot of time together and I know them pretty well, etc. (Not in the sense that they vent to me, I can just usually figure it out to some degree.) And obviously it's the same for Fern with my relationships.

Fern has always been extremely awesome about accomodating and supporting me - I'm 100% the messy partner, I've struggled with insecurity about their other relationships at times but Fern has consistently just been happy for me and so generous with making time for me to be away.

Dramatic drumroll..... until now.

No, the worst part of this is that they're still awesome. This year I started seeing someone new, basically around the same time that I brought up some completely poly-unrelated issues in my relationship with Fern that really hit them hard. We're working on it and we'll get through it, but it's clear that the bad timing has created a lot of insecurities for Fern in regard to my new relationship. It very clearly bothers them when I spend time with new person (okay let's call them Yarrow), and they're noticeably less generous in supporting this relationship than they have been in the past.

And when I say "less generous", it means "they hesitated a second when I asked if I could borrow their laptop (for videocalling Yarrow, which wasn't explicitly stated but they could infer)" and "they politely brought up that they would like me to plan it better the next time Yarrow visits" (the last time Yarrow was here was pretty chaotic due to some unrelated stuff, but I definitely leaned on Fern to pick up a lot of slack to make that work). So like. Still awesome! But they look sad about it! And it hurts so much to see them sad and unhappy due to my actions. It makes me feel guilty about prioritizing my time with Yarrow. And then I feel annoyed about feeling guilty. And then I feel guilty for being annoyed, since basically I have created this whole bad situation for Fern.

I'm not going to end my relationship with Yarrow, that's not on the map and wouldn't really help anything, and Fern wouldn't ask me for that ever. And obviously I'm checking in with them continuously to see if there are ways for me to accommodate them, along with making an effort to show how much I appreciate them and make time for us to focus on our relationship. And all that stuff. But I don't know if it's enough. This is a completely unknown situation for me, and I'm worried that I'm doing the wrong things and creating other problems down the line.

If you're still with me this far.... I think my actual questions are: what can I do to deal with the guilt I feel about hurting Fern, and prevent it from poisoning my relationship with Yarrow (or Fern)? And are there other things I can do to support Fern through this?