"We have lost the spark" sounds clear.
It usually is not.
One person means sex has become rare.
Another means they no longer feel admired.
Another means the relationship is predictable.
Another means they are carrying resentment.
Another means they miss affection, not intercourse.
If you do not define what is missing, you can spend months trying to fix the wrong thing.
Separate desire into parts
Score each area from 0 to 10.