How do you fix a spalling concrete retainer wall in a bedroom?

The problem that you face is external rather than internal in that spalling usually happens where the internal surface freezes but that seems so unlikely

There are a number of epoxy-resins and similar concrete repair products on the market as since I'm in England I can only give you generic keyWords to search on

If it is possible, I would advise you to install some sort of land drainage around the property like a gravelled trench to drain away the excess water that settles in the wall That should remove the cause of the spalling