Recommend a back massager?

When I overdo it, I often get terrific muscle pain and spasms in my back, mostly in the thoracic area but also in the lumbar. They last for hours. Sometimes the pain lasts for days; right now I’m on day 5.

I need a back massager that I can put behind my back either sitting or lying down. I want either no heat or optional heat. It has to be something that I can operate myself, not requiring someone else to apply.

There’s a ton of them on Amazon, but it’s hard to judge what’s good.

Do you have a recommendation for me?

If so, many thanks.