Advice needed

Hello all,
So I really don't hobby at the moment and haven't for awhile but still lurk around here because it is interesting and who knows maybe one day I will get back into the hobby.
Anyway why I am posting is because my so is really wanting to explore her submissive side to sexuality after reading the fifty shades of grey books. I know its becoming quite a phenomenon right now as I have read about this quite alot recently.
While I have had my sure of limited kinkiness in the past (toys, blindfolding, etc.) I have never really been into bdsm. A little spanking during sex is about it and not much of that. My personality is really the exact opposite of it. She has really taken to it and I am trying my best to accommodate her. However it has gotten to her wanting me to whip her until she is bruised so when she sits down she can really feel it. I have tried this a few times for her, but I really am not aroused by it and the bruises are not sexy to me at all. She looks at them in the mirror and it arouses her tremendously.
Has anyone had experience with this? Any advice for me on ways to get over the trepidation that I am having with this? It has gotten to where she is asking for this with every sexual encounter.
I once dated a girl who was into masochism. almost a pain slut. I would crop, cane, spank, paddle and whip her until she was marked up but it wouldn't do a damn thing for me. she wore the marks like badges of honor and showed them off to all her friends......some even called me up asking me to do the same to them.
as soon as I stopped the pain in the bedroom play, she broke up with me saying she could get straight sex from any random boy
be careful how you trend is my advice here I guess