I am studying a subject where I failed twice. So, it is normal that I am really scared of this. And everytime sth difficult appears, i get mental block. I feel I might not be able to solve this. But the reality is that now I have tons of resources for learning this subject whereas at the time I failed I had none. But that mental block is making me learn this subject not so well. I am scared to spend time learning due to this mental block. How do I overcome this practically?
If you failed it twice then you need to change something about how you study. Perhaps it is that you don't study it enough. Perhaps it is that you are using ineffective techniques. Some people, for example, try to leave study for the night before an exam and then try to learn everything at once. It doesn't work.
Many universities have an office that has people that can help you with this sort of problem. If so, you should visit them and take their advice. In most places (not all, sadly) the professor would be a good source of information about what you might do.
One thing that helps is to ask a lot of questions, in or outside class. Another is to take notes effectively. But probably the best way is to do more than is strictly required in terms of homework, exercises and writing and such.
I've had a few students who had their seemingly first positive educational experience in one of my courses. They did it by visiting my office whenever I had open hours and asking questions. Often the same questions. But it was that engagement that got them through.