I'm a recent graduate with a PhD in mathematical logic; I can totally relate to this problem of having a non-standard background. Before Grad School I went to a liberal arts college, where my math major consisted of something like 9 courses. When my advanced studies began I felt totally lost. But you have to ask yourself: Do I really have the time and energy to commit to a high level exposition of physics at this point? The answer to this question will depend on whether you intend to specialize in PDE's. If the answer is no, then I believe you should buy some good popular explication of the topics you're covering. Unfortunately, I do not know what such a book would be, but having read extremely good expositions of several high level mathematical concepts (Prime Obsession; Unknown Quantity; "e", the Story of a Number, and Incompleteness) I have some confidence that a book of similar quality may exist in this area. Of course, if you are intending to specialize in PDE's, it will be worth the time and effort you will need to invest in reading a serious text. Even so, keep in mind that you can never learn everything, even about a small subdiscipline. My advice is to find a particular area of PDE's, become an expert on that, and get out of graduate school as soon as possible.