You need to think carefully about what it is to 'fancy' someone. It changes over time. Of course it does. Have you seen any old couples (really old, I mean, not just 50's or 60's even) who sit around holding hands, smile secretively at each other, help each other around and so on? (If not, look around you a bit more. We're there!) Think a bit about these oldies ...
... Do you think those old people fancy each other? Of course they don't in the way young people do; they don't get the immediate physical/emotional/mental response they perhaps once had; they see their partner's saggy arse, wrinkled skin, arthritis-knobbled joints and all the rest. Time tells on us humans. But, somehow, they do (they really do, trust me on this, I know!) definitely fancy one another still, albeit in a different way.
OK, you're not there yet. Not by a long chalk. But you're on a path that (if you're very lucky) is leading there. And (again, trust me on this) it's somewhere very nice to be if you can manage it.
Now think of how there might be a continuous change in what 'fancying' someone means if the original heart-stopping-lost-my-appetite-I-love-you-you're-so-beautiful 'fancying' is to change, little-by-little to this other, deeper, more-considered-but-if-anything-stronger-I-love-you-you're-still-beautiful kind of 'fancying' that you hope (if you have any sense) to achieve some day.
Can you fit how you do/don't 'fancy' your partner just at this moment on that sort of continuum, with a bit (perhaps a lot) of thought? If so, stay together. Work at your relationship. (All relationships need work; how could they not?) Talk. Think. Talk more. Have lots of sex. Talk about sex. Etc., etc. ... If not - and if you really can't see any way of getting back onto such a continuum - well, you might need to think again, maybe put the relationship to rest.
Good luck!