You mention having studied single variable calculus and linear algebra. Green's Theorem is a result from multivariable/vector calculus, a course commonly expected to have been taken as part of an undergraduate engineering degree. To get started on vector calculus, you might consider Robert Ghrist's excellent set of video lectures: Calculus BLUE.
The Cholesky factorization is perhaps not emphasized in all introductory linear algebra courses. Have you seen the LU decomposition? If you're familiar with that, it isn't a big step up to Cholesky. See Gilbert Strang's video lectures on MIT OpenCourseware to refresh/learn topics in linear algebra.
Warning: do not assume that just watching videos is enough to understand; it's a starting point, but you'll still need to do exercises and check or get feedback on your answers. For this you'll need a book or other resource with answers and/or a tutor. Note also Michael Field's advice in his preface to Essential Real Analysis, Springer 2017:
On occasions I advise students in my analysis classes not to spend too much time reading mathematics texts. That view is based on my own experience—an effective way to learn mathematics is to do it, play with it but generally avoid spending too much time reading books about it. Reading a mathematics book can give a veneer of superficial understanding that dissolves the moment one tries to use the theory described in the book. An analogy might be learning carpentry, plumbing or a foreign language—knowing the theory is important but not that helpful; knowing how to use the tools is crucial. That takes time, practice and serious effort.
It isn't clear to me how much knowledge of probability and statistics is required for your particular type of engineering, but "mean, median and mode" are indeed unlikely to be enough. At the very least, find out whether your courses need more probability or more statistics and focus on getting up to speed with the basics of the one that's used most or at least first.
This is to get you started and help fill in some gaps or areas where your knowledge might be rusty. It will take a lot of work, but once you have strengthened your foundations, you may find that you are "fit for Engineering" after all. As others correctly say, though, there are no shortcuts*. It will probably be a bumpy and painful road for quite some time to come.
* That said, see Captain Emacs' answer; sometimes a more targeted approach can save you time.