> to never fully complete them and see it all the way through
Others who do have the persistence to see something all the way through, are simply, lucky! Lucky to have the drive. But there are things you can do, to improve your drive and determination, namely drugs. Go see a psychiatrist who can help you with that. Once you get a feel of what it's like to stay focused on something for a long time and see it through, you'll want to do it more often and naturally will learn some non-drug techniques to get there.
Some other things you may have heard numerous times, but doesn't help to repeat: have a hobby (could be as simple as taking care of plants), exercise regularly, preferably in activities that involve other people (I find top rope rock climbing to be perfect for this; squash also comes to mind).
> I'm stuck in a rut, wanting to better my skills and earn a good amount of money but unable to concentrate, riddled with brain fog, and unsure of my future.
This further tells me your issues aren't sprung up from your skills, but rather at a more basic level.
I can also recommend Sam Harris'es waking up meditation app which can help immensely with navigating your mind and better understanding how to focus your attention on things that matter the most.