This is my problem:
If an Englishman is speaking, I can understand him well.
I've read many English novels and stories, I can understand English movies without subtitles, and when I need subtitles, I only activate English subtitles, not those in my native language.
But I have problems with speaking!
I can't speak English fluently..
What you need to do is find native speakers (or other people with a near-native level of English) and engage in conversations with them. To make this more concrete:
Good luck!
As the people above have said, the best thing to do to improve your speaking skills is to find people around you or on the Internet with whom to speak.
In another discussion, I have given a link to a Discord multilingual vocal server (like TeamSpeak) where it's possible to find a number of diverse language rooms :
Ling & Lang: https://discordapp.com/invite/DUCVm
In France we have a famous youtuber named "Linguisticae" who has his own Discord server which works like "Ling & Lang" :
Linguisticae: https://discordapp.com/invite/YkMzzuN
Verbling is also a good solution for communicating vocally and visually through its webconference service (it's for free but you will have to create an account):
https://www.verbling.com/community
Good websites to get in touch with a lot of people :
Polyglot Club: http://polyglotclub.com/
GoSpeaky: https://www.gospeaky.com/
From the time you start creating a network of fellow speakers everything becomes easier.
And don't forget that the best thing for speaking a lot with people is if you share the same interests; for example video games :D
In my personal experience I can tell you that it is difficult to learn a language. In my case I'm not English and my native language is Spanish but I have improved a lot with practice, practice and practice. Use your imagination, be creative and have fun with English. How to speak fluent? Well, that's not easy but not impossible. Try speaking with others, people who is learning English too, friends, etc. Is like everyone says, practice every day but just be creative and the most important thing is, have fun with the language!
As @Manuel said you have to practice a lot. Speak to other persons who are learning English. Speak with native speakers too.
If you are on your own use your imagination. You said you watch a lot of movies and novels. Imagine a scene that differs from the original novel. Play the roles of all characters and speak the dialogs to yourself. Best to do this when you are alone.
If imagining scenes is difficult for you, try repeating scenes you saw in a movie or novel. However using your imagination helps you to a greater extent. It has no limits and it can help you more if you can imagine and practice real life scenarios. For example, imagine a meeting with your business clients and talk to yourself in the language you are learning to speak. It gives you confidence in real life too.
I think the cause is clearly lack of practice.
As for tips, if one has social anxieties or really doesn't have someone to practice with, speaking to oneself helps a lot. I know because it worked for me with English. As teenager, I did have 4 years of English courses on a private school and then years working with computers/programming languages, reading a lot of literature and comics, watching movies and listening songs, but speaking to no one.
Try to have all your inner conversations in English; when you're angry or joyful with yourself and have something to shout, in English; having that fictional dialogues with someone else (as rehersal before talking to them; or as "I should have answered that" after talking with them), in English; and so forth.
Try to speak it loudly as much as possible but having the conversation only in thoughts is ok too. We don't want friends, family, neighbours, hearing us talking in weird tongues with some imaginary speaking partner -gesprekspartner :)
And then, difficult to pronounce words or sentences, those deserve special treatment: repeat them aloud till it hurts!
When the time comes and you really have to interact with other folks in English, things will flow smoothly.
I agree about practicing with other English speakers. However, if this is not possible for you, just try to think in English. I mean, one of the key skills for bilingual people is that, as you move forward, you don't need to translate everything in your head before speaking (which I find is one of the main reasons why people struggle with fluency), so try to think in English, even if it sounds lame.
For example, if you need to make a groceries list in your head, do it in English. If you need to reflect on your day, do it in English. If you need to search for something on the web, do it in English.
A higher level of receptive competence, i.e. listening & reading, & a lower level of productive competence, i.e. speaking & writing, are pretty normal & yours is a common question, which is essentially, "How can I develop my productive competence more quickly?"
It clearly sounds like you've already done a lot of receptive practice & that's an excellent & necessary foundation for developing productive competence. You're already off to a great start.
I think the difficulty with developing productive competence is best explained by Processability Theory (Manfred Pienemann). A very simplified example of this is, when we listen to or read a sentence we typically process the parts more or less in the following order: First noun phrase, final phrase (noun, adverbial, prepositional, etc.) & then finally the main verb phrase. As soon as we can infer the meaning, we stop processing the sentence & move on to the next one. More than 9 times out of 10, our inferences are correct & deeper processing is unnecessary. What this means is that we typically don't process enough of what we listen to or read to be able to reconstruct the form (the "grammar") within them.
What this also means is that practising speaking & writing (in meaningful ways) are necessary but insufficient. We also need to attend to & process form (the "grammar"). This is what Merrill Swain established in the early 80s & Mike Long has coined the term "focus on form" (FonF) to refer to language learning activities that attend to it.
It's worth emphasising that (pragmatic) meaning is a core/essential element in FonF. Without meaning, it's impossible to make form-meaning connections. So language should always be studied with its socio-pragmatic context made clear, i.e. Who's saying what to whom & why. What's the (social) purpose of this instance of language use?
Here's some video presentations of critical reviews of typical classroom language learning activities which I think would be very helpful to both learners & teachers of foreign & second languages: https://www.youtube.com/playlist?list=PLCvyUoftI84vFgjodGPGYSGjA2uco0L49 Hopefully, these will make clear which activities are most likely to improve your productive competences.
I hope this helps & good luck! :)
