It's not a disaster if you don't learn.
Spaniards put themselves under a lot of pressure to master English. Why this is, I don't know. It's not like they speak a language that is useless beyond its borders. It's not like there's so much trade between Spain and English-speaking countries – companies from English-speaking countries do not dominate employment in this country.
There is a lot of embarrassment and fear about being left behind.
There's a lot of jealousy directed towards people who have mastered English. Worse, there's a lot of resentment towards people who make an effort to improve their English. Why this is, I don't know. But it's sad to see it happen.
I don't want anyone to think that they're totally incapable of learning English. (Spanish students of English say this a lot, but I have yet to meet someone who shows it's true.) But being able to do something, and having the time, the determination and the interest in doing something well are different things.
If you decide to learn something, and you say that you're doing it to improve your future, remember that you basically enter into a contract with two people: yourself, and the teacher. The teacher's responsibility is to create a positive, educational environment that helps you get you where you want to go, literally and/or figuratively.
It's YOUR responsibility to do what you can to make sure that happens.
And if it doesn't happen, and you don't learn to speak English perfectly...ask yourself this: What, exactly, have you lost? You can probably still communicate fairly well. If you had to go to London tomorrow, you'd probably do all right in the airport and at the hotel and in a restaurant. So what if you'll never be able to negotiate a multi-million dollar deal, or act in an Oscar-winning movie, or any of that stuff? Do you want to commit to the amount of work and study and practice that a person needs to be able to do that effectively?
Trust me...there's no shame in answering "no." Unless you're marrying someone who speaks English, unless you're going to work for an American company or you're moving to an English-speaking country, "good" is good enough.