Although your smoking is not the only possible explanation for your problem, I suspect that this is either the cause or at least a factor. There's only so much an MRI can tell you.
I have a feeling that there's been some subtle mayhem going on when you smoke - the brain cells getting into bad habits. Anyway, you heard the man's advice - stop smoking.
Instead of thinking of the brain as a machine, think of it as a very complex organization or community. A machine is always the same unless something gets broken but your brain undergoes changes according to how your live. For example, if you learn to play an instrument, your brain will undergo structural changes as you do this. it's called plasticity. You're literally shaping your brain all the time.
The good news is, the way you live can improve the structures of your brain involved with memory and problem-solving ability.
Article - "Mindfulness practice leads to increases in regional brain gray matter density"
I like to talk about mindfulness and mind-body therapy a lot because it can help with depression, which might be part of your problem. You might be at least mildly depressed. I've written a lot of depression answers, with advice from experts and you're welcome to read. You might be interested for example in the work of Steve Ilardi, the therapist/researcher who headed the U of Kansas lifestyle-depression project. This is an easy way to energize your system - physically and mentally.
Mindfulness-Based Stress Reduction, which has been validated with hundreds of studies, teaches meditation so that people can learn to use mindfulness in daily life. Palouse Mindfulness is a free online version of that program, taught by a certified MBSR instructor. There's also a popular app, Headspace.
I suggest starting with the MBSR Body Scan (below), which is very relaxing.
A study with college students showed improvement of grades with mindfulness training. They said they found it easier to concentrate while studying.
Simple, practical methods of dealing with a memory problem -
I don't always have time to write something down, but in the blink of an eye I can create a mnemonic device with some object, like putting a bill I want to pay soon near the front door. This is so easy.
Writing things down helps of course. You might get yourself a weekly planner. Keep a To Do list in it.
If you're getting along without your drug, you should have something like a hobby to get your mind involved - something other than just sitting and watching TV. Find something that makes you use your thinking and memory. Your mind needs exercise like your body.
And have some fun once in a while. No law against good music.