My ex works in the substance abuse field and one of her personal interests is the effect of drug use on the brain. It is well documented that substance abuse causes brain damage, but in the past it was thought that that damage was permenant. However since the advent of more advanced brain imagery scanning it has been shown that, through abstinance, the brain actually heals itself. Some damage is permenant, but a signifent portion, especially the frontal lobe area, regenerates itself.
While it is true that substance abuse destroys brain cells, apparantly when you stop using the nuerotransmitters create new connections with functioning cells to "restore" function.
So in answer to your question about what you can do? Keep doing what you are doing. Our bodies have an amazing way of fixing the terible things we do to them. BTW, it's usually a matter of months and years, rather than days for this to happen. You gotta figure you've been abusing for 20 years, so it won't happen overnight, but it will get better if you don't use. Take care.