Because all the words and grammar are different. I thought that was obvious! :))

Learning a language isn't hard...You just need the time, patience, and the dedication to be able to do it. Most people get frustrated far to easily when they try to learn a new language. When learning a new language becomes to overwhelming, just slow down, take a break, then try again. Don't give up!

It isn't hard. Keep practising and you'll get used to it. What's making it difficult for you is that you think it difficult.

Because it's hard to force myself to think and feel in foreign language.

It isn't. :) It wasn't hard to learn your native language, was it? It just takes some getting used to.