Let me drop this little proverb here for you:
塵も積もれば山となる
The base of your mountain may be from those boulder-sized 冬, 緑, and 花 kanji, but even the dust-sized kanji you learn later on build up to a lot of knowledge.
Anyway, it sounds like you might want to try reading some manga or books. The variety beyond mahjong might help you regain some motivation, and maybe you’ll find a new hobby that you enjoy, just like how you found mahjong. If you do want to try this approach, check out Master List of Book Clubs. There are a lot of book clubs in this community where you can read with others and learn a lot. In fact, the Absolute Beginner Book Club just started a new manga today.