That is the point of spaced repetition systems (SRS) in language learning. The assumption is that you WILL forget everything you've learned pretty quickly unless someone or something reminds you of it regularly. There are tons out there, find one that works for you and stick with it. I find they are great for remembering vocabulary and short phrases, not so great for everything else.
As for how I can tell I'm making progress, it's when I try media/content made for native speakers. I know I'm intermediate when I can follow what they're saying and pick out words and phrases with the help of English subtitles. I consider myself advanced when I can do the same, but with subtitles in the language. And I'd say I'm fluent once I don't need any subtitles to follow native content. I focus more on watching and understanding things rather than speaking, so these are 100% my own personal standards.

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