Megan,
Yes, you have chosen a long, tough song. But I have to disagree partly with DHeun: it never changes tempo, only mood. You can do a Nightclub Two-Step or Society tempo foxtrot or combination of both to this, with more lively movement, spins and turns during the "Crazy/Crush" refrain. I would stop at 2'04", or 2'26 if you want to do the refrain twice.
This will take some practice and some dancing ability. It is not a beginner's dancing tune. Find a skilled and understanding instructor. If it's just too much, try this:
1) Learn a simple rumba box step and get the rhythm down so your feet can do it in your sleep.
2) Now forget the box step and just do the rhythm with your feet, with your fiance, keeping a good-looking frame as you move around any way the music takes you. Use this for all the sweet romantic slow parts.
3) Do a 70s-80s style freestyle during the louder/busier "refrain" parts.
Good luck!