How are you studying for certifications and why are you forgetting everything after 6 months?
If you are studying for certifications properly-- by working through examples in your own environment, for example, or by reading up on various aspects of the product/ language you're interested in-- you should be retaining the knowledge for more than 6 months. That is particularly true if your day-to-day job involves at least similar tasks to those that the certifications cover. On the other hand, if you are studying for certifications by trying to cram from test prep books (or, even worse, brain dumps), it's no surprise that you're not retaining the information. If the goal is to learn something, you'll want to avoid cramming for the exam and you'll want to focus on learning the material knowing that the more thoroughly you understand the material, the easier the exam will be.
If you're exaggerating about forgetting "everything" and the problem is really that you're forgetting some esoteric bit of trivia that happened to be a topic for the exam but that you haven't had any reason to work with for the past 6 months, that's likely a different topic. Whether or not it's related to a certification exam, no one remembers everything they read about 6 months ago if they haven't had a reason to use the information since then.