My postdoc advisor wants me to work on a topic that I don't have a proper theoretical background in. How to proceed with this? My PhD involved using a commercially available software to carry out simulations. Now I am pursuing postdoc at another lab. My current advisor wants me to develop new complex models for which I don't have any academic experience in. I have 3-4 months before he starts expecting outcomes.

I am panicking as I don't have theoretical background to do the job. This will involve studying 2 courses right from scratch. (continuum mechanics, non linear finite element analysis). This, I have to self study right from basics and I am feeling that I will fail. 

I never lied on my resume. I knew this project will involve more rigor than what I had done before. But now, I feel that I should have done more coursework during my PhD. My undergrad was in different  field, so never took classes for these subjects. Now, as a postdoc in a lab, it's impossible to do actual courses in a university. I feel miserable.

Do you have any suggestions on how I should proceed with this?
I started my PhD in computer science in 2002. My last prior formal education was a master's degree awarded in 1975. Everything I knew about the prior quarter century of developments in computer science I had learned by self-education from books and papers, as well as practical experience and a couple of industry courses. Although I did take courses during my PhD studies, my dissertation research depended on basic ideas in linear algebra that I learned from books.

I find it hard to understand deciding in your 20's that you are too old to learn new basic material from books, papers, on-line tutorials, and other self-education.

To proceed, I suggest discussing your need for more basics with your advisor, not in a spirit of expecting failure, but asking for advice on what you need to know and how to go about learning it. Your advisor may recommend books, or a course you can audit.

For your long term future, it would be better to learn now how to learn without courses, and get over your current block. Avoiding it by collaboration will just leave the block in place to stop you the next time you need to add to your basic knowledge.