I've been using Duolingo to learn French mostly.
I feel I learned a lot when I started using this website. However, I'm getting to this point where all I'm making are small mistakes that leave me losing the hearts for the lesson. I know the vocabulary, and it's getting incredibly frustrating for me because I can't seem to fix these issues. I want to move on, but if I can't get this stuff right, why should I?
It's mostly with prepositions when translating French back to English. If I do a lesson enough times, I end up memorizing the answers, but then I get careless sometimes and miss others!
Do I just need to suck it up and do it?
The program can't distinguish between little mistakes and serious ones. If you use another preposition in English but keep the meaning, report the sentence and suggest to accept your answer. If you got the meaning wrong, practice more.
If you are completely stuck and you feel that this method does not work for you, here is a little "cheat" for you: write down every sentence you get with its translation. Then use your notes when you attempt to pass the lesson for the next time. Writing is better than copying and pasting because it makes you remember something. I did this with some of the French lessons and I'm not ashamed of that. Instead of going through a lot of frustrating drills, I passed the lessons and learned those prepositions later when I began to read in French. Prepositions and adverbs are much easier to learn in a good context. However, for conjugations and other stuff like this drills are very effective.