Confession of a Trainer's Secret Kink

I've been working as a personal trainer at one of the most prestigious gyms in town for almost three years now. I love my job, and I think I’m pretty good at it too, clients come back because they see results, not just because I have that six-pack from doing squats till I drop. But there's this thing about me that has nothing to do with the gym, that’s been eating away at me for a long time now. My partner and I are happy together, but we both have our different interests, mine happens to be something she doesn’t share or like at all: breath control play. Don't get me wrong; she knows about it because I’ve told her since the beginning of us being together. But every time I bring up wanting to explore this side of intimacy with her, she gets uncomfortable and changes the subject. So here’s how it goes, whenever we have a moment alone where things start getting hot and heavy, just before climaxing, I’ll hold my breath and let out little sounds as if struggling. It turns me on like crazy knowing that I can push myself to this edge and she can’t. But when I try to talk about adding it into our sex life more directly, all the ways that could be done, she backs off. The other day, we were having a really intimate moment, she was on top of me in a deep, dark place of passion where things seemed open and honest. When I tried holding my breath again as part of it, something inside her seemed to shut down, like walls going up. She stopped, looked at me with this pained look that I couldn’t read. Since then, I’ve been obsessively searching online for ways to talk about it without scaring her off but haven't found anything helpful yet. Does what I do fall under BDSM or is it just a personal kink? Is there something wrong with wanting to control my breath during sex as a way of controlling the experience and feeling more in tune with myself? Is this normal? Do I bring it up again, or let sleeping dogs lie? Should I seek out others who share these interests for a safe exploration, knowing it won’t happen at home anytime soon?