To answer the "why is it hard?" question, I tried to collect my own experiences at http://www.acooke.org/cute/WhyandHowW0.html - not sure I did a good job, but the main conclusion was that you underestimate how important experience is in avoiding errors.
To repeat what others have said in answer to your more general question - solutions to "real world" problems include more than a single call to a primitive. So you need to find libraries that provide a higher level API, like parts of NaCL http://nacl.cr.yp.to/, Google's keyczar http://www.keyczar.org/, etc.
Even for simply encrypting a string with a password - https://pypi.python.org/pypi/simple-crypt which is what I talk about in the first link - I needed three things: key strengthening, the encryption itself, and an HMAC. Making those work well together was harder than I expected (at least 5 bugs harder...)
To repeat what others have said in answer to your more general question - solutions to "real world" problems include more than a single call to a primitive. So you need to find libraries that provide a higher level API, like parts of NaCL http://nacl.cr.yp.to/, Google's keyczar http://www.keyczar.org/, etc.
Even for simply encrypting a string with a password - https://pypi.python.org/pypi/simple-crypt which is what I talk about in the first link - I needed three things: key strengthening, the encryption itself, and an HMAC. Making those work well together was harder than I expected (at least 5 bugs harder...)