Whats wrong with my Holden VS Commodore transmission?

I have a 1990 holden vs commodore and have lost 3rd and 4th gear. I have to drive it in 2nd on the console because in drive it does nothing and in 1st it just revs out.I drive at 4000rpms in 2nd gear will this stuff my engine? and do i need a new transmission and if so is i hard to install myself?