I think that everybody, at some time or other, has problems with pronunciation. I believe this is because we spend our whole lives speaking our native languages and then, when we want to learn something new, we have to start using our mouths in new ways. The mouth is like a musical instrument used to create sounds by flexing the lips, tongue, jaw and facial muscles to vary the size and shape of the mouth as we speak. We also use the throat, vocal cords and air from the lungs in the production of sound. If we spend our lives producing the same set of sounds all the time and then suddenly we have to start to make some new ones, it’s only natural that we are going to have some difficulties. For example, I have no problem with the English word ‘internationalisation’ but I always struggle the Spanish version ‘internacionalizasión’, even though the words are almost the same.
The way the letter ‘R’ is pronounced is English is not the same in Spanish or in German but we can get close enough. Some languages have letters that are not used in other languages. Look in a Spanish dictionary for words beginning with the letter ‘W’ and you will be limited to imports such as ‘walkie-talkie’, ‘webcam’ and ‘whisky’. Common enough words but perhaps not used often enough to help master the production of the ‘W’ sound at all times.
Speaking a new language goes beyond learning grammar and vocabulary. We need to learn new sounds and sound combinations too, especially when we want to improve fluency and words start to ‘join together’ as we speak. We can practice these with familiar phrases like ‘fish ’n' chips’ to distinguish between ‘sh’ and ‘ch’, for example, and we can even do facial and breathing exercises to help us to ‘loosen up’.
Try taking a look at the TED talk by Julian Treasure called ‘How to speak so that people want to listen.’ Towards the end of the talk, he gives some recommendations for exercises to ‘warm up’ the mouth. You might want to develop your own ‘oral gymnastics’ to improve the quality of the sounds you make.