I have puzzled in my job, can you do me a favor?

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I get a job from a foreign company, firstly it make me very pleasure. but now i have a great dilemma, i find that i am very tired everyday. i want do this job well and make my Boss admire me. This idea make me crazy, i try my best to do this job, i still worry i can't do it well, especilly my boss kick out 2 guys because their work is not good recently.nowadays i even don't believe my work capability, i am afraid that the boss will kick out me in the future. Can you give me a favor and give me some suggestion. Thank you very much.