Why was it difficult for China to change to a modern lifestyle?

One should be cautious when proposing questions in this way, because it assumes that a “modern lifestyle,” or modernity, more generally, is a universally understood and benevolent state of society to which all people should mutually aspire. Furthermore, such a question might also be construed as somewhat demeaning, as it implies that there is no inherent value in traditional Chinese philosophies, materiality, or ways of life.
Modernity in a historical sense is a concept peculiar to eighteenth- and nineteenth- century Western Europe, which European imperialists then exported to all other parts of the world. It was a specifically European historical development, with a specifically Eurocentric value system that judged any other civilization that did not exactly resemble the Western European model as atavistic, barbaric, and socially and culturally inferior.
China was heavily occupied by a large number of European colonial powers in the late-nineteenth and early-twentieth centuries; the British maintained nominal control in Chinese port towns such as Tienshian and Hong Kong, the French held concessions in southern China along the coast and eventually in Shanghai, and the Russians disputed constantly with the Chinese over ownership of territory in Manchuria.
Each of these European great powers brought with them ideas of modernity and a firm resolve to bring the Chinese empire up to the standards of modern European civilization. The British, for example, initiated a sweeping set of hygienic reforms in the north and greatly frowned upon the use and sale of opium. With these reforms came a call to put an end to centuries-old and highly venerated Chinese practices, which were often tied to the ethics of the ancient teachings of Confucianism and Daoism.
For its part, China did engage in a large-scale process of internal reform. Intellectuals like Liang Qichao looked to Western models of government in order to find a suitable replacement to the increasingly obsolete Chinese monarchy, which had become isolated from the general public by the end of the nineteenth century. The Chinese also engaged in a healthy east-west trade and adopted a number of European technologies, including railroad designs, modern vaccines, and published newspapers that were available to the public. In doing so, Chinese statesmen were willingly letting go of much of their heritage.
Either way, one must take care not to put too much emphasis on the specific European innovations in China and take this as a sign of general progress. Instead, becoming “modern” really just meant replacing one set of ideologies and customs for another in the face of intense external international pressure and decaying internal politics.