Some people are complete a-holes.
I ride or run with people and always keep on an eye on slower people to make sure everyone keeps together.
If people want to ride or run fast then they should state so-to avoid super noobs/unfit or injured folk joining up and killing themselves.
Your partner told you it was a steady run. For her it was a steady run-for you it was gentle jog to rehab your knee.
If she knew that then she is bit self focussed. If she didn't then you should have told her of your planned rehab run.
Whatever, just plan your rehab if you are healed and go on your own.
Why not get some easy mates to join you on a Bicycle Ride!
Your mind created this-not hers. I would not see it as competitiveness I would just train by myself till I'm in shape then join her.
Some people are natural selfish people who mean no harm intentionally.
My GF is competitive too but we also communicate-sometimes lol.
Get over it, train on your own, ride with pals and get fit again.
And if you're still miffed-talk or just outpace her in run to boost your self-esteem. Bit childish though.