I think what you're trying to say is:
Nihongo no benkyou wa muzukashii noni tanoshii desu.
(にほんごのべんきょうはむずかしいのにたのしいだす。)
Alternatively, you could also say:
Nihongo no benkyou wa muzukashii desuga, tanoshii desu.
(にほんごのべんきょうはむずかしいですが、たのしいだす。)
:)