Your sentence literally means "I speak difficult (as in high level) Japanese. This is because I'm a quiet person." Option: "私は日本語を話すのが難しいです。恥かしがり屋ですから。" ( = "It is difficult for me to speak Japanese because I get shy easily.") Option: "私は日本語が話しにくいです。内気な性格ですから。” ( = "It's challenging for me to speak Japanese because I have a timid personality.") These are just some ideas. Of course there are many other ways to express this.

No, in opposite. It's saying you can speak difficult Japanese. 私にとって(for me) 日本語は(Japanese) 難しいです。(is difficult.) or maybe 日本語を話すのは(to speak Japanese) There are some possible translations for "shy person" but I recommend you 「口下手(くちべた)」, which means not good at oral expression. 口下手ですから。