What is one way of reducing massive forgetting after immediate learning?

You are experiencing a ‘forgetting curve’ which is the loss of information that you have just studied, the sharpest decline occurring in the first twenty minutes after study and this would appear to be your problem.

If you wish to retain information learned for many years, you need to practice Spaced Repetition which is not the same as repeating over and over, it is reviewing what you have learned at spaced intervals - just at the point when you might forget it. In your case, it would be 5 minutes, 10 minutes, 1 hour, 1 day, 5-7 days, 1 month, 3 months, 6 months, and 1 year but alter this schedule to what works for you.