I've been studying statistics for almost a year now, but I feel like I can't advance any further. I've tried by attending advanced classes, but even when I already know the topic I still don't understand a lot of things. For example, sometimes the professor shows a proof of some theorem and adds or deletes something and I really don't know why.
I've tried to study more high-level texts (such as Casella-Berger "Statistical Inference") but I wasn't able to understand most of the content. What I'm asking here is something (maybe an intermediate book or online course?) to go beyond my actual level before I dive into a more advanced book.
For example, as I said, I cannot understand Casella-Berger. Can you suggest me something like "OK, study this book and that book and after that you'll be able to understand Casella-Berger".
EDIT: I'll try to clarify my question as best as I can. I have a background in economics (but I think it can be generalized to any non mathematical-statistical-physical subject) and I want to apply for a PhD in statistics. My professor suggested to me the Casella-Berger book (saying: "but maybe it's too hard for you...". He was right, it was too hard). I've spent months studying and I feel like I gained nothing because I've understood something like 10% of the content.
The books I've studied so far are:
Statistics for Business and Economics by Newbold, Carlson, Thorne
An Introduction to Statistical Learning by Gareth James, Daniela Witten, Trevor Hastie and Robert Tibshirani
a few specific topics from various sources
The reason I want to learn all of this is that I'm reading a lot of papers and they are very obscure to me. I can understand the introduction sometimes, but when it comes to the methods part there are statements, proofs, etc. and I feel completely lost after a couple of minutes. I can't see myself writing a paper in the future if I don't get more knowledge in statistics. Maybe I'm wrong but, in my idea, if I'll be able to tackle the CB book I'll be able to understand (almost) any paper.
Thanks Glen_b -Reinstate Monica and whuber for your suggestions, I've read the other topics and they've been useful. My request though is more about a path than a single book. Calculus is, for sure, something I should start studying.