It is tougher for adults to learn a language, we do know that. Once psychologists thought that adults could never learn a second language. We now know that even adults have brain plasticity, not as much as developing children, but it's there. If we did not have plasticity, we would not be able to learn anything new. Once in a while a book on Linguistics will argue that a native speaker can never fully learn a new language. My personal stance on that is that while we do not fully comprehend how children are able to grasp language, other than the fact that their brains are developing at incredible rates during the critical period, we therefore can not fully understand whether or not a non native speaker can fully grasp a new language.
Nevertheless I still struggle learning German, and to learn a new language in adulthood is a pain in the neck. That doesn't mean that someone cannot do it. But telling yourself you cannot do something before even trying will not get you anywhere.
Take for example this study conducted by a graduate student in 1939, completely unethical by today's standards
In 1939, University of Iowa graduate student Mary Tudor began an experiment with local orphans, warning them that they were showing signs of stuttering and lecturing them whenever they repeated a word. The children became acutely self-conscious, and many began to stutter, fulfilling the theory that “the affliction is caused by the diagnosis.”The article is actually about how one of the subject wrote her a letter years later accusing her of being a monster and how the study ruined his life. Jesus Christ!
Nonetheless the study goes to show that "the affliction is caused by the diagnosis."If you say you cannot do something, you cant. Tell yourself you know you can. Not only that, but use positive thoughts and motivation not only but your self, but others! What have you got to lose?