2021 Prime Limited drivers seat

No problem here

 

Saw some cushions on sale at a local Costco. Worth a shot....

 

I haven't noticed any difference with my 2021 Prius Prime than with my 2020 Prius Prime, and they are both comfortable.

You obviously have some shoulder and/or back injury and sitting in a car for a prolonged period is aggravating it. Physical therapy or a chiropractor might help, along with exercise. Good luck and I hope you feel better.

 

Yes ! I'm not sure if this is the problem with your seat, but....
In one of my older cars I've had a seat where the SRS (air bags) system sensor in the seat poked out a bit, just enough that I could feel it when I brushed my hand across the seatback.
This also caused the SRS dash light to come on in my car. I was able to shut the SRS dash light by pressing on the SRS sensor in the seatback.

I fixed it by buying another used seat for the car.

If you can feel something in the seat that doesn't feel right to you and the others that have driven your car
I'd recommend you find another car like you have and see if you feel the same thing in the same place as you do with your car. Than you'll know pretty much for sure.

edit: oopps I was looking at the OP's profile car 2010 and missed the thread title about 2021

 

Just in case, have you tweaked (softened) the lumbar support? Round button on the side...

 

It could also be a pea.

The story tells of a prince who wants to marry a princess but is having difficulty finding a suitable wife. Something is always wrong with those he meets and he cannot be certain they are real princesses because they have bad table manners or they are not his type. One stormy night a young woman drenched with rain seeks shelter in the prince's castle. She claims to be a princess, so the prince's mother decides to test their unexpected, unwitting guest by placing a pea in the bed she is offered for the night, covered by huge mattresses and 20 feather-beds. In the morning, the guest tells her hosts that she endured a sleepless night, kept awake by something hard in the bed that she is certain has bruised her. With the proof of her bruised back, the princess passes the test and the prince rejoices. A huge wedding takes place in the palace. The prince couldn't believe that he found his true princess. Only a real princess would have the sensitivity to feel a pea through such a quantity of bedding, so the two are married. The story ends with the pea being placed in a museum, where according to the story it can still be seen today unless someone has removed it.



The Princess and the Pea - Wikipedia

 

Since it's a new car (I missed the title and looked in your profile instead, sorry about that) I'd recommend you try another Prime at a dealer to see if the seats in the second Prime feel the same to you as yours. It is a low sitting profile and if you're used to sitting higher it can be uncomfortable at best.