Hi, in my opinion it is not really difficult, but it does take a lot of energy and time. So, be committed. Once you are here, if possible, take a daily private class (if only 1 hour) at least for the first few months, to keep the rythm. In class, focus on those topics that are of interest to you. In your daily life, note down things you would have wanted to say but couldn’t and ask about it in class. Put the words from your class and any other words you run into in a system to memorize them (I use Trainchinese flashcards but there are loads of similar systems), in this way you are forced to learn and relearn the vocabulary that is useful for you. Use the lessons to learn about grammar rules, idiomatic expressions and to gain fluency. Most importantly, do not be afraid to speak, even if you only speak very little. In my experience, people here are very patient and friendly and do their best to find out what you’re trying to say.
Online in preparation, you could start with Rosetta Stone, I am doing it now and find it really useful. But that, too, requires a daily commitment. Good luck and have fun in Chengdu!