Loss of mental abilities after abuse?

I abused substances and medications to improve my mental performance for a year. I stopped 3 months ago and I feel dumber and dumber (Almost as if I were becoming incapable)... Is it really happening or is it just an impression? At the height of my abuse, I was considered practically a walking machine... Now I'm not even close to my shadow. What can I do to get back? Without abusing it anymore? Neuro plasticity?