This will happen with any student. We commit mistakes and that is human tendency! The question is what are we going to do after committing the mistake? Are we going to learn or give up? This is a principle that applies to every walk of our lives. A sincere student will try to learn from the mistakes rather than becoming discouraged and frustrated.
Especially when we are learning a topic in physics and we are trying to do some problems – you will be able to do some depending on your understanding till then and you will not be able to do some because your understanding is not sufficient yet – so you make mistakes. We should understand that mistakes will help us to learn and understand the concept in a much better way. So see mistakes as a “Learning opportunity” rather than as an “Inability” in you. You will be able to make a positive progress in your learning of the subject.