I remember a word during my reviews, but not when it is used in context. How can I fix this?

The way I think about it is two things need to happen to really learn words in another language so you can use them: you need to realize the missing word, and you need to study each word.

Traditionally, one would try to say something, translate the word, then study that word later. However, with spaced repetition, you do the study of the word ahead of time, and 'activate' it in context later. You will still need to create the connections to real usage in your brain, and this is often going to be the case of needing the word and not recalling it, only to later realize the word was already in rotation in your spaced repetition.

For a lot of words, one failed recall is enough to activate the words permanently. However other words may require several failed recalls before they activate. The good news is that this is easy to practice at home - just try using the vocabulary around the house, speaking to yourself, spouse, room mate, pets, etc. Anytime you use a word you're learning in a novel, off-the-cuff manner, you're "activating" the vocabulary just as well as in real conversation.

Do I need to worry about fossilization?

No.

Your success in a language is going to be directly correlated to how comfortable you are making mistakes, and how comfortable you are making assumptions. Lots of influencers, tutors, and language teachers are trying to sell their product by warning people that if they learn things incorrectly at first, that these mistakes will 'fossilize' and become permanent. This isn't just a vast misunderstanding of what fossilization means, but an insult to language education as a whole.

I assure you, you have fossilized mistakes in your native language. I can also assure you, that there is no mistake so fossilized you cannot recover from.

Most of the people warning about fossilization will also promote mass-input in the same breath. Anyone who has learned from mass-input knows that it's largely a matter of making assumptions about words and phrases, most of which will be wildly incorrect to start with, and continuously reworking and refining the meaning in your head.

How long does it take to get fluent?

Years.