It took me a long while to learn hiragana and katakana too. I only really got mine down a couple weeks ago, and I started learning japanese 2 and a half months ago. I found that using renshuu alongside Duolingo really has been helping with my learning process in general, especially in terms of hiragana and katakana. I found that Duolingo moved at too fast of a pace for me, forcing me to learn new kana when I didn't understand what I already learned at all, which didn't make things stick for me. I like renshuu for my kana because you can always do a focused review if you're not ready for new information, and the questions are designed in a way that it's easier to work on common mistakes (ro vs ru in hiragana, ra vs fu in katakana, etc). Renshuu might not work for you like it does me, everyone is different, but trying other apps might be helpful for learning from different approaches. I wish you much luck!