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.