HI - tried googling Bennachie as I havent done it but I struggled to find a "hill" - is it Mithers Top you mean or one of the others? see this link
http://www.forestry.gov.uk/pdf/BennachieLeafletMap.pdf/$file/BennachieLeafletMap.pdfTo be honest coming from that angle if its Mithers top (although its a small vs Nevis) doesnt look too steep but isn't very flat either - going by the contours..and a ridiculous amount of playing on google earth trying to get a look at it..
Nevis by the pony track is a long walk and at times given its size and the "steps" on the way can be tough going (on the knees mostly!) but in general its not "steep" although that's subjective i guess depending on what you've done before. I don't remember any point on it where falling back would have done me much more damage than a sore bum although there are a few high steps I guess.
Suggestion is you try a few other hills... before nevis I did Conic Hill, Ben A'an, then Ben Ledi plus a few wanders round Callendar Crags and these will all give you a better understanding of what the terrain at height can be but none are "scary" in any way.
Hope you find your Hill legs because once you get into it you will love it.. and good luck with Nevis!
P.s Im a great believer in pushing yourself just gradually beyond your current comfort zone always make you enoy life a little bit more - so go do some hills and I'm sure you will not only gain the confidence but relish it!
After that - Hypnotism?