This is another thing, too, where a learner's current level of proficiency determines what words are and aren't in "frequent use." When you've only been studying the language for six months and it's still a struggle to understand and produce even very simple sentences, many words will seem obscure because you just aren't taking in or producing Japanese at a high enough level to encounter them frequently. As you gain proficiency, and are able to read/understand "real" Japanese at a deeper level and greater speed, you give yourself more opportunities to encounter more complex words more and more often.
For example, I wouldn't recommend a first-year Japanese student who is still struggling with the て-form of verbs to spend excessive time trying to memorize a word like 可能性 (just for a random example) -- it would be pointless, because even if they could remember that it means "possibility", they wouldn't be capable of understanding or participating in the kind of conversation (or reading the sort of text) in which 可能性 would turn up.
I guess what I'm trying to say is: don't sweat it when you forget a word. It'll stick eventually, but not until you reach the level where it has a reason to stick.
I'm not quite sure what you're saying here. Learning a language is cumulative simply by nature, something which you touch upon in your comment. It's not like you learn one structure and then never see it again -- you see it in different contexts, and in conjunction with new structures/new vocabulary/etc., which serves to reinforce the old material and allows you to deepen your understanding of the various ways in which it can be used.
It sounds more like you're just saying that classes move too fast -- you're going on to new things before you've had time to master the old stuff. Maybe this just means you need to devote more time to studying, or to reviewing past material.