Why is it so difficult to find a topic for my thesis research?
Your supervisor or potential supervisors should be able to help - and I don't necessarily mean by talking to them.  First of all identify potential supervisors whose (recent) work you like and (mostly) understand. In the first instance look at their recent papers, and particularly concentrate on any points they raise in discussion or future work.  In particular do a SWOT analysis and note what are the best and worst features of the approaches, and whether you can see opportunities for different, even interdisciplinary, work to be brought to bear.

To the extent you don't understand anything, don't keep reading and rereading and struggling with it, go back to the earlier work and the citations.  Try go be back to the originators of the concepts you are lacking or find difficult, and the commentators who clarify the insights behind the formalisms.  Look for conference and workshop papers for these early developmental versions of work, rather than the erudite and impenetrable journal versions.