I just found out about a better methodology after already writing thesis - how should I prepare for my defense in 3 weeks? I have written a thesis that seeks to analyze the ideological thinking of a newspaper through its editorials in a political situation during a certain period of time. I couldn’t count on my Thesis Advisor and I have written my thesis by myself. 

My methodology is simple. In short, I made a compilation of information about the problem, then I selected 20 meaningful dates during the political climate in question, so I had a sample of 20 editorials: 1 per each meaningful date. Then, after checking out the editorials I found 8 units of analysis and made a Content Analysis to find the expressions used on the editorials.  That's how I found the newspaper’s judgments, attitudes and attributes about diverse events and characters (important people) related to the political situation. These represent the variables or categories that would demonstrate the newspaper’s ideological posture.

Nevertheless, after applying this methodology (contents analysis) in my thesis, I read about Critical Discourse Analysis (CDA). This is the most appropriate method to analyze the ideological thinking of a newspaper. Even that, my Advisor approved my thesis and I must defend it in three weeks. I am afraid that in my defense someone asked me why I used Content Analysis instead of ACD, being my thesis a qualitative investigation. I do not know how to answer appropriately. I would like to read your opinions and recommendations, my friends.
This situation is one of many that demonstrate the key role of an academic advisor and why I would never suggest anyone attempt a research program as a novice without one.

The specific content of your research (i.e., the merits of content analysis vs. critical discourse analysis for your research question) is off-topic for Academia.SE, but at this point you should find an advisor in your field to ask for advice.

If you cannot reach your thesis advisor, then maybe someone else on your committee can step into that role. Your advisor has approved your work, so either they have not read it carefully (and have failed you miserably in their advising duties) or this problem is not as serious as you may think (i.e., your choice can be justified despite your oversight of another technique in the field).