I can't install ruby on rails on my mac - Sorry I know it's been posted 1 million times before

Have you looked at RVM? If not, check it out. It makes installing/managing Ruby versions a synch, as well as gems.

I use these two pages to get RVM, Ruby and RoR setup and haven't had
any problems

https://rvm.io/rvm/install/
http://www.unfoldingcode.com/2011/05/using-rvm-to-install-rails-31-on-ubuntu.html