Has anyone broken a hand bone from carrying too many groceries?

I’ve NEVER heard of a hand breaking from carrying too many groceries, but that doesn’t mean anything other than I didn’t hear about it!!! Since you have broken your hand before, it is somewhat normal to have persisting pain afterwards. (I had a foot operation 50 years ago & when I overdo it, it will still let me know) It could also be your body telling you that you’re overdoing it & to back off!!! Maybe baby the previously broken hand at little bit & not overuse it so bad. Make it a conscious habit to carry the lighter bags in that hand leaving the heavier bags for your good hand!!! I can only use one hand, so when that shoulder starts complaining about me overworking it, I back off some & make a few more trips or I get out my handy little rolling cart & let it do most of the work!!!

I believe in listening to our bodies as they usually don’t lie about what they need (or don’t like)!!!

You could try something like this. Or find totes with a shoulder strap.
https://www.amazon.com/Pankia-Handle-Grocery-Shopping-Carrier/dp/B07Y4RM371/ref=sr_1_5?crid=1JK93ENCMVIQN&keywords=grocery+bag+handles+for+plastic+bags&qid=1656166046&sprefix=Grocery+bag+handle%2Caps%2C150&sr=8-5

I broke a bone in my right hand a little over 50 years ago which the doctor called a boxer’s break. I’ve never hand any pain or discomfort from it but I know a broken bone will increase the likely hood of arthritic pain. That may be what you’re experiencing.

Never ignore pain! Your prior injury is playing a big part in the pain you are feeling.
Carrying multiple plastic bags together poses a challenge for fingers, wrists and elbows. You can certainly strain your delicate finger joints or arms lifting too much weight awkwardly. If you have to use plastic, carry one bag at a time in each hand. It would actually be better to ask for paper bags instead of plastic, and carry them using both arms under the bag rather than grasping them with your fingers. A gallon of milk weighs about eight pounds. Lifting it out of the refrigerator with only your fingertips or in a plastic bag held by just your fingertips can torque and twist the joints.
Also, Never ignore pain.