In the beginning of our relationships neuroscience, our endocrine system and our nervous system practically do all the work for us. It's biological auto-pilot, psychological bliss aka the honeymoon phase. We aren't the only species that does this - in the beginning of a beaver relationship they mate constantly for days! But after we become accustomed to someone, our body dials back on how exciting we find this person and shifts to companionate vs passionate feelings. You can come up with your own reasoning for why mother nature does this BUT the point is after a year or so of interacting with someone sexually, our desire for them often shifts from automatic to manual drive.
So what does this mean? It means you gotta put in the work: You gotta find new ways to add excitement to your relationship, you have to engage in sex because you want to not just when you ferociously "need" to, mainly, you and your partner have to stop waiting for lust to just happen.
Don't stress you've done this before! Remember when you first got that guitar and you were SO AMPED to practice everyday? Then after awhile that thrill passed but you didn't give up because you realized the guitar was JUST as awesome as it had always been and the only thing that had changed was your attitude towards it. So you resisted the urge to quit, you kept at it and now you are the next Jimi Hendrix...Okay not really but I hope you get the point.