I'm starting to feel it's impossible to learn a different language and speak FLUENTLY. I'm gonna keep going though.
I failed languages all the time as a school kid, and I became fluent in Spanish after less than two years. Don't give up! One thing that really helped was to stop watching and reading things in English as much as is possible, and only do those things in Spanish. Of course, it's hard to do that if you have a job or a family, but whenever you get free time, try to immerse yourself in Spanish.
It is not at all impossible to speak another language fluently, and many, many millions of people (both children and adults) around the world are living proof of that. The average child in any country is generally fluent in their first language before they begin school. They may not be able to read or write that language, however, they understand most of what they hear and have little or no difficulty being understood by others when they speak. Realize that they heard that language for THOUSANDS of hours before they began speaking it themselves. You do not have that luxury of time and immersion in acquiring a second language. So, be patient with yourself. You have mastered English, a notoriously difficult language to write correctly, for instance. Your choice of Spanish is made easier by the fact that many vocabulary words are shared by both languages, and many native speakers of Spanish live throughout the U.S. You also have easy access to television and radio programs in Spanish. Many resources in Spanish are easily available online, too. Again, be patient with yourself and just "keep going!"
I used to think that. I was awful in languages in school but when Duo came along I thought: "Why not give it a try. No teachers over my head, no report cards home, there's nothing to lose." Well, after four years I'm still going strong going from language to language and now with the new upcoming format which lets you progress further, there's no stopping me. And it's fun!
It's not. English isn't my native language and I fool people into thinking it is all the time. ;)
It just takes a long time to achieve fluency.
After you get the basics like grammar down, you should start immersing yourself. If you can't speak with people in your target language, just read books or watch movies in it. You'll increase your vocabulary and learn to get words you don't understand from context and over time you'll get a feeling for the language and instinctively know what sentence construction sounds "right" or "wrong" and may be able to use it. Then start writing more, find penpals on the internet or go to Spanish messageboards, travel and speak with natives etc.
Not impossible if you stay dedicated! My only source of learning another language for free is this app and Memrise. I never thought I'd see the day where I, without much an effort or thought, form Spanish phrases and identify objects in Spanish. I'm also to the point where I'm able to understand words/phrases here and there in a Spanish conversation. I was never able to do that before. Now, if there's Spanish dialogue in a movie or show, my family looks to me and asks what it is being said, haha.
As of now, I'm between intermediate and advanced in Spanish, a beginner in American Sign Language, Japanese, and Korean. Just from dedicated practice, although I have more of an advantage in Spanish since I took it throughout high school and become proficient in writing and reading it. Don't give up. You can do it! Good luck on your language endeavors.