Another thing to consider is your kitty's pattern of relaxing. Most kitties have some sort of post-eating ritual. One of mine eats/grooms/naps. The other eats/plays/grooms/naps. So maybe delay the shot until the end of grooming time, just before she goes to sleep. She may be less wiggly at that point. Or plan a play session just before test/feed/shoot, so she's more relaxed and has some of the wiggles out before you try to inject. Or get the syringe ready, and then take a few minutes for chin scratches, cuddles, and kisses before picking up the syringe to inject. Cats don't wiggle 24/7. Most of the time they are relaxed, so careful observation of your cat's natural rhythm, and then using that to your advantage may help.
Also making sure that
you are very very calm at injection time. Cats get wiggly either when they need to play or when they are anxious, so being careful not to communicate any need to be anxious helps to settle them down. That is part of my earlier suggestion to take a break if it's not going well so you both have a minute to calm down before trying again.
There is always a way to make it work, it's just a matter of unlocking your cat's mysteries and finding that way. A few other things that have helped either my cat, or cats I've read about here: reciting poetry, singing a song quietly, listening to music, putting the cat in front of a nature show on TV, giving the cat a favorite stuffed animal to cuddle, placing a pot of cat grass in front of him/her, placing the cat on a favorite windowsill so they are busy watching the bird feeder, and on and on.
They do make kitty calming meds, so if it really feels insurmountable, a conversation with the vet might be helpful. There are also feliway plug-ins that work for some cats, as well as Rescue Remedy and as you already mentioned, catnip - just make sure she is
eating the catnip and not just smelling the catnip. They each impact the brain differently and eating catnip calms them while smelling it amps them up.
This video is also interesting. It shows how to use food to train a cat to take injections. One of mine isn't at all food motivated, but I've used the same technique, but using his brush instead of a plate of food since he is super motivated by being brushed:
Sam has come a long way in staying calm for his injections. And I've made progress in reading him better, so I know when he's off and I'm going to need to shift his mood before I can successfully inject. And also in reading
myself better, so I know if I'm in a hurry or otherwise communicating anxiety to him and can step back and settle myself before I try to inject. Some days just
are harder than others, and on those days I need to slow down and make sure we are both calm and ready before I even try.
Just hang in there - you'll figure out the combination of things that work best, and as those become a habit, it really will get easier.
