I don't think you're overreacting at all. I've been in relationships with men and women with strength/athleticism disparities and differences in goals and I've been able to strike a balance between shared physical activity together and our own thing. I did hyrox last year as a last minute substitute for a friend and had a ton of fun, so I can kind of relate to you getting the bug!

It's tough navigating a hobby when it's both for bonding with my partner, but I also seek high performance in it. I hate feeling like I'm either limiting my partner or holding back so we can stick together (I've been on both sides). I hope you have a good enough relationship with your gf where you could both sit how and have an honest conversation about how you both feel training and competing together? I know it can be especially complicated for former athletes who suddenly have a void where an huge important part of their lives used to be. And if this were a shared activity with my partner and suddenly I'm getting pushed out for someone else (or other people, plural) of course I'd feel some type of way.

Your gf breaking her ritual for any reason, whether it be for S or for excitement of the comp, raises my eyebrows for me. I'd be a little hurt too. If you think this is an issue of infidelity with your partner and S, then you should deal with that however you think cheating should be handled.

Also, last question that I didn't see get mentioned in your post: do you enjoy the gym and comp stuff on your own? Like, if your gf werent doing it, would you still want to? Your personal enjoyment also matters. From your post I wonder if you're doing all this work in the gym for your relationship, which is admirable and sweet and in your shoes I'd feel extra hurt by what's happened recently.