Can you first explain what you mean by 'venture' because it is not normally a word that is used as a noun, it is a verb, something you do, not something you have.
Taking that word out, I would delete the first sentences and say:
Last week, I was given an offer from an American company. It really was a rather satisfying job I have always wanted to do, but higher language skills were required. Although my experience in the techniques of the job were enough to meet the requirements of the position, I was still worried about my skills in English expression. I have spent a lot of time listening and reading in English, and I can understand what others say well, but the biggest problem is I cannot describe my thoughts and opinions well. For the last two days, I have been anxious about this matter. I have been really worried about my English skills.