Suggest a book where the protagonist goes into a virtual world, but they think its real?

There are lots of ‘goes into another world’ stories. Its the ‘but doesn’t know it’ part that I’m looking for.

What if you could pay to live any experience you choose, without any knowledge that it wasn’t real, and then return to your real life, with the memory of the experience intact?

What if the visitors to Westworld actually believed it was real, until it was over?

What if in Project Hail Mary, Ryland Grace paid for a ‘memory experience’, but he didn’t know it was a virtual experience until it was over, and got to keep the memory?

I’m looking for a book/story where the main character goes into a fantasy/virtual world. Either they paid someone to ‘send’ them there, like “Eternal Sunshine of the Spotless Mind”, or maybe it was involuntary.
The other world is not real, but it seems real to them as long as they are there. They have no memory of thier ‘real’ life. As they are living the experience, ‘clues’ about the real world sneak into the virtual world, until they eventually figure it out. Maybe the storyline goes back and forth between real/virtual.

by ezeeetm