This is not a problem with your ability to learn. The reason you forgot the subject you studied 12 months ago is because it was 12 MONTHS ago. I would suggest focusing on time management. 4 months is more than enough time to learn these new technical subjects if you make a solid game plan. Put in the hours. Work more than just 9-5. Schedule 1.5 hour blocks of learning. Find a friend to check your current understanding of the topic. This has worked well for me in the past.