You need to make a schedule and stick to it. Get organised
do what you like the most. and continue study along with that. relate your lessons with real life examples
there are people proudly sporting an F-, a B- isn't bad...you want to get back to your prime though? you need to first leave your pride out of it...you can't let failures put you down. Think about it this way: a man goes regularly to the gym, he decided to try dead lifting, but failed miserably...does that mean he needs to give up, go home and never go back? no. His body is simply not ready yet, through more rigorous training, he'll be able to life that weight and more! however, if he decided to accelerate his improvement using steroids or not respecting his body's limits, he may get an injury that takes him off the gym for good. Same thing with you, your knowledge isn't vast enough in that specific field, but that only means you need to find a methodical way to study even more efficiently...perhaps find a study buddy or a group of serious people to help you focus, otherwise try a work and reward system...keep in mind that there is no perfect study system for all, but there's a special perfect one for each...you just gotta find yours! Once you do, you'll hopefully become able to accelerate in your study...keep in mind however, avoid all-nighters, they are the same as lifting more than your body weight and will cause physical issues, avoid also working without breaks...I did it and...the ending wasn't pleasant. Anyway, lift your chin up, trust in yourself and try a different angle...your enemy is despair, and as long as you keep trying, you haven't failed yet.
you are not becoming dumb you are just becoming normal