I've always had a poor short term memory but a good long term one, I suppose, but it helps if you do your best to focus on what your trying to achieve and set things in a series of steps. Don't allow yourself to stop focusing or get distracted. A good thing about the brain is it can be altered to some degree. The brain has plasticity. It's a bit like a muscle; the parts you use will get stronger and over time things you felt were mentally difficult will be easier. This is certain to happen if you use those parts often.
My short term memory is improving; mainly because my job is at the right level of challenging for me, it's a little difficult but not beyond my capabilities and also that I slow down when doing things and understand that I don't pick things up as quickly as 'normal' people.
I'm not sure if that's the same for you. I'd say that anything you strive to improve upon can only get better. 
_________________
We become what we think about; since everything in the beginning is just an idea.
Destruction and creation are 2 sides of the same coin.