What is the biggest challenge you feel that you face in becoming fluent in your target language?
Speaking is the hardest skill to master, I find.
But the challenges in getting to fluency are (for me)
a) laziness -- it takes a lot of hard work
b) a lack of focus: I like to study several languages at a time, and since I don't neeeeed to learn any of them, I advance slowly in each of them instead of getting really good at one
c) a lack of time: with other languages I already know and want to keep up, and all the real life things one has to do, there isn't actually the kind of time left I'd need to really immerse myself in a language and get to fluency
Try to determine and then achieve some intermediate goals for yourself. For me, Duolingo's gamification has worked wonders, but not everyone likes having a green cartoon owl track their performance and boss them around.
Depending on what your skill level is, could you set some goals like "read one book a month" (you can always start with really simple ones, and be prepared that the first pages and books are going to be hard going), "read one Wikipedia article in your target language on a topic you are already knowledgeable per day", "go through this textbook within three months" (and then reward yourself with something after that!), "watch one movie in your target language a week", etc?
Anything that keeps you going, really. The road to fluency is really long, and it's important to score some victories on the way.
For me, it's resources. I have determination and time to do it, but I struggle to find enough resources to help me to lean to be fluent in the specific language that I'm learning. For Russian and Italian, I have resources. For Finnish, however, it's just Memrise for now. I'm hoping to expand my Finnish learning if I can find a few other good resources besides it (like books and audio/visual stuff). Keep on learnin'!
My biggest challenge is hearing in the language. I have a hearing problem that cannot be solved with a hearing aid. That means I'll never fulfill my dream of sitting down and watching a telenovela in Spanish. It's challenging because I still need to find the motivation to continue. So far, so good, but at some point I'm going to hit a brick wall.
It really depends on both your personality and the language in question, though.
For some reason, I don't mind sounding like an idiot when speaking Spanish, but I am much more scared of speaking French, although my French is better and I have more experience communicating (mostly successfully, even) in French.
I can speak French more or less fluently with regards to pronunciation and grammar, but what I am now in the process of doing is just growing my vocabulary and trying to remember idiomatic expressions and the like. It is especially hard to be able to speak and understand everything in French because there are several different registers of language such as: soutenu (fancy), courant (normal), and familier/argot (informal/slang).
One of my main problems ( other than vocabulary and grammar ) is trying to understand a language without making the constant effort to translate whatever I'm listening to into English.
Something else that I have a big problem with that I don't see discussed all that often ( or maybe I'm not looking in the right places ) are sentences that could be used mid-conversation. Such as : "I was wondering about...", "What do you think about...", "Umm...", "In other words..., "In fact...", or "Well according to...". Does anyone know where I can find a collection of these phrases in other languages?
Finding material in Russian that's simple enough to understand with a limited vocabulary. In Romance languages one can often read things about history / politics / economics because of all the common vocab. That works substantially less well in languages further afield, and children's books, even picture books, can be hard because they're often about animals (which you don't know the words for) doing everyday-ish but not actually that common things: gardening, lounging around on the porch, which you also don't know the words for.
At the end of the day, the answer I think is that you're just going to be looking up lots of words. Immersion was great as an interface to support that (even though it was still tons of work that very few even tree-finishers seemed willing to take on). I miss it.
Very difficult not to give up but to keep an interest. I have no particular use for the language I'm learning now, so am very prone to giving up. I have already lost interest on another platform (italki) some months ago. I saw a video on YouTube by "LingoSteve" who, I seem to recall, recommended plowing through a Duolingo as a first step toward a new language. He as a second step recommended simultaneously listening to an audiobook and that very same book as a paperback, listening while you are reading. I once learned to read German by plowing through the book "Ganz Unten" by Günter Walraff (very much talked about at the time), where I started at page 1 manned with a dictionary, but this crude method is not something I would recommend anybody... https://de.wikipedia.org/wiki/Ganz_unten