Probability is a very fuzzy concept for beginners. Starters are always intimidated by formal notations and seemingly disconnected ideas.
The key to the rapid progress in probability is to start by understanding counting and combinatorics. Often professors will tell you not to study combinatorics very deep as his course doesn't require that much. But, the truth is, combinatorics is the most crucial part of understanding probability.
When you are done with combinatorics, study the following books sequentially.
The first one will help you to develop basic understandings. The second one will teach you formal notations and theoretical aspects of Probability. The third one will complete your knowledge of probability and statistics.
If you find trouble understanding concepts during studying the books, go to YouTube and watch corresponding tutorial videos. There are thousands of them.