Of course, of all the qualities that predict success in graduate school, preparation is one of the biggest. One of the people I started my PhD program with dropped out after one semester; not because he wasn't capable and interested, but because it had been so long since he last took classes, he found he was no longer prepared for the work required.
If you're planning to take a graduate degree in a subject that you have not studied recently, you might consider taking a few classes as a non-degree student first; this will let you ease back into the academic world and see if you're still able to focus on your learning as you could in the past. If you struggled as an undergraduate student and never learned good study habits, you're unlikely to be successful in grad school, where the expectations are much higher; again, it's probably worth taking classes as a non-degree student to see if you can handle it. If you don't want to spend a lot of money before jumping in, you can often audit a class, which tends to be cheaper: in this case, you go to class, but you do only the work you choose and do not receive a grade. (Even after you start a degree, auditing is a good way to be exposed to material you're interested in but where you may not have the background required to take the course successfully).
The good thing is that if you decide to stop, you've hopefully learned something, and if you choose to take the class for real rather than auditing it, you can probably use that credit towards your degree if you decide to continue.