I'm sad. (man ghamginam)

Man mikhaaham be zabaan-e farsi mosallat beshavam. :'(

This is correct but it is probably not what you are trying to convey. This would be more appropriate in a one on one conversation as a response to a question. Like if I was talking to someone about Farsi and he asked me what I want to do. My response would be: man mikhaam (mikhaaham is shortened to mikhaam when one talks) be zaabaan-e Farsi mosallat besham (same with beshavam, it is shorted to besham when talking).

Now if I wanted to write a formal sentence indicating my desire to master Farsi, then I would write: man mikhaaham be zabaan-e Farsi mosallat basham. I would not write 'beshavam' because it would suggest a future want whereas you are trying to say this is what you want now.

Overall you did very well. I stretched this a bit and tried hard to find something to correct for you.