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。)

:)