Without knowing details, it's hard to say... What I've experienced and observed, is that most relationships hit a phase once you've gotten past the initial excitement/lust part and you think you're really comfortable together but don't actually know how to communicate very well and set boundaries for yourself, which brings me to my point: it sounds like you might just be spending too much time together.
The popular narrative in our culture is that we get is that once you're coupled up, the more time you spend together, the more committed you are to each other, which is actually a super insecure and unrealistic perspective. In the happiest long term (25+ year) marriages I personally see, they're giving each other a lot of space. Like, your significant other wants to go on a trip that sounds awful to you? Encourage them to go with a friend instead. If your relationship is on solid ground, you shouldn't have any trust issues with that, and visa versa.
It takes some time and communication to get to that point, and there will always be give and take, but just because you're in a long-term relationship with someone doesn't mean you should be monopolizing all of each other's time and energy.
Also, to be crystal clear, I'm not talking about infidelity or sex with other people. It's hard to be attracted to someone who's been annoying the hell out of you all day, so again, giving each other space and maintaining time together for the times and activities that you (mostly) enjoy together will help keep the fires burning.
(Been with my wife for almost 16 years)