What are you trying to say?

I don't see any big problem with grammar. However, it doesn't sound very professional, and the last sentence sounds more like Chinglish. Overall, it's not clear to me what you are trying to say here.

I don't want to discourage you, but your English needs a major overhaul. If I were you, I'd spend every minute to improve language skills, or you won't land a job you want.

If you want the recipant to know that you can work long hours, I'd say:

Dear XXX,

I am very interested in working at XXX. I am ready to meet any of the challenges requested by the job even if it means working long hours.Should you have any questions regarding my applicaiton, feel free to contact me.

Looking forward to hearing from you.

Best regards,

A&F