I think one of the hardest things about kanji is that they have multiple pronunciations, but the cheat code to figuring them out is by learning the pronunciations in context. Don't try to memorize kanji by themselves: learn WORDS that use those kanji and practice using that vocabulary to cement the kanji in your memory. If you're using flash cards, at the very least put example words using the onyomi and kunyomi on every card, and then have cards for the OTHER kanji in those words as well.
Then you can start leveraging the following structure to ask questions and learn new kanji in Japanese:
One of the most powerful tools moving forward is XOのX or XOのO... XOのXは何 (のX/の漢字) ですか?
A:あの…関心の「かん」は何の漢字ですか?「感じる」の「かん」ですか?
B:いいえ、「関係」の「かん」ですよ。「関する」そして「関わる」って事ですね。
A:そうですか。「関西」の「かん」と同じですか?
B:そうですね、「関西」の「かん」です。
A:へ〜、なるほどです。
This structure lets you ask in Japanese which other words use that kanji, which helps you build mental connections between those words. It's a bit like learning the greek or latin roots of words in English; once you learn the constituent parts of "anthropology," "philanthropy," "geology," and "misapprehend," you can start drawing connections and guessing the meaning of newly-introduced words like "biology," "misanthropy," "geothermal," and "thermodynamics."