Wow, a lot of what's been posted on this thread is resonating with me, and it's reassuring to know that others struggle with this too (although I wish we didn't have to). I really worry about this in our marriage, although I don't think I'm in exactly the same boat as some, and have no plans to voice my concerns anytime soon.
Like the OP, I feel I've changed a huge amount since I got married in my early 20s (I'm 44 now) - and I had a really uncharacteristic spell last year when I was quite sad about not really experiencing single life, and fearing that maybe I never will have the independence I sometimes really long for. I guess after 20+ years of marriage I feel quite cynical about it, and I resent some of the compromises - and many of the things that once attracted me to DH have faded long ago, admittedly some of these are superficial and he still has a lot of very good qualities. We just aren't a lovey-dovey, romantic couple, and that's ok with me I think... I don't wish for it with anyone else... I just think it's the way many long-term relationships settle down probably. Comfortable companionship instead of sparks flying IYKWIM?
I'm not really enjoying this sort of shitty mid-life era either, I think it's wearing us both down, we're tired and there are so many decisions to make, holding down jobs, parents to care for/worry about as well as the kids... it's a bit relentless, so it's not surprising that our relationship might take a back seat. It's not brilliant, but it is what it is.
Do we love each other? Yes. Are we 'in love' - I'm not sure... Am I 100% happy in our relationship? No, probably not, but who is? And would I be happier if we separated? - almost certainly no. It would just bring a whole other set of problems - more, probably - and blow apart the family unit we've worked hard to build. I don't think I could do it to the kids, or to our parents tbh.
I do worry about the mental toll of muddling through the tougher times and repressing the worries/fears sometimes, but on balance I think I'm in this until something changes for the worse, or (fingers crossed) things somehow improve with time.
Sorry for the essay! But it's quite cathartic, as others have said, to get this out - it goes around in my head often and I do struggle to work out the reality of the situation vs what I'm blowing out of proportion in my own head. There's a certain amount of post-pandemic, perimenopausal madness going on in there just now I think so that's another reason to sit tight - I don't trust my hormones affecting my own mental state sometimes!
OP I do hope you figure things out for the best - ultimately, you're the only ones who know how to make yourselves happy (or happier, at least), whether that's staying together or not.