We're talking very personal things here, but here's my view of this.
Mentally and spiritually i rhink are related, but different. I would think of mentally as being able to cope with the visualization of new concepts, change the way you think about moving etc - purely brain work.
Spiritually touches more of your views on life - in a more emotional way. Comes from the soul if you like (anybody know what 'soul' is?).It means, for example, that you would need your dancing to keep you fulfilled, that everything you do in the rest of your life would be 'anchored' to your need to dance - what you eat, even the job you have (in extreme cases). It certainly means that you truly love dancing and couldn't love without it, because at the highest levels in any sport great demands are put on you mentally and physically - it's just bloody hard work. You need that inner insatiable desire to keep you going.
THeese are just my views. I'll be interested to hear some others