Hello! I am a english teacher and a brazilian portuguese teacher as well. Of course I have been a student (and still am). I am passionate about languages and have studied english, french, spanish and korean. Of course I am not 100& fluent in the latest , being an interpreter in english and portuguese (which means I have achieved a native level in both to be able to perform the profession). I can, though, communicate well in spanish and read/hear french ok and now I am in love with the korean language.
Well I know that learning languages as an adult is way harder than when we are kids. I say that from experience, but that does not mean it is impossible. You need to dedicate to the language you want to learn eevry day, even if it is just listening to music or studying for an hour. Attending classes is also essential and even though technology is nice nothing replaces a real teacher. I honestly can't learn anything watching videos online as it is soooooo boring and annoying. I like real face to face.
Dedicate yourself in all 4 aspects of a language (writing, listening, speaking and writing). Listen to songs and "think" in the language. Also read, watch shows and be in contact with the language as much as you can.