I've gone through Atocha many times and never found it particularly confusing. But I can offer a few tips:
There will be a security check before the platforms for long-distance trains. Far easier than at an airport, but it can be a choke-point. Leave time for it. For an 11:50 train, I'd try to be there at least 30 mins early. You can get a coffee inside security if it all goes quickly.
The specific platform number may not be posted until around 20 mins before departure. Then there may be a rush to a long line to have your ticket scanned and go to the train. If you're in the line you'll get on the train.
7B is your seat number, but before that there's a "Coche" number for which train car you're in. There's a 7B in every car, so on the platform look for the correct 'coche' number and just board there.
There are Walkarounds of the station on YouTube, if a bit of orientation might make you more comfortable.