In order to differentiate between nationalism and patriotism I turned to the trust Oxford English Dictionary because everywhere else the definitions were literally the same exact words. For myself the only differences between the words were connotations that I thought were subjective because of my own individual contacts with the words in specific contexts.
"1. a. Advocacy of or support for the interests of one's own nation, esp. to the exclusion or detriment of the interests of other nations. Also: advocacy of or support for national independence or self-determination.
[in smaller subtext] Whereas patriotism usually refers to a general sentiment, nationalism now usually refers to a specific ideology, esp. one expressed through political activism. In earlier use, however, the two appear to have been more or less interchangeable."
(Oxford English Dictionary online at http://dictionary.oed.com/cgi/entry/00321451?single=1&query_type=word&queryword=nationalism&first=1&max_to_show=10)
At least the OED confirms that it's not just lack of knowledge that has led us all to think of nationalism and patriotism as synonyms. Apparently they have essentially been synonyms for a long time and only just recently have become more differentiated. So, patriotism is a more general feeling of support for one's own nation (its interests, etc.) even when doing so may harm other nations. Nationalism is that but for a specific ideology in support of one's nation's interest.
Does anyone agree or disagree with the OED? I would add that nationalism has a more aggressive connotation and patriotism more defensive, but I think that is, like I mentioned, subjective.