how to learn engling by yourself

I can see from your question that you are engaged in sales and marketing. Learning English, especially oral English, is actually very simple, and confidence is the key. There are too many examples like Li Yang. Speak more, watch more and listen more. Don't be afraid of jokes from others. Thinking about mastering a foreign language will give you motivation. When it comes to majors, I think you just need to remember some professional words, because many times when you say that word, others will know what you want to express. There are too many textbooks for self-study English in the market now. Just find one that suits you. I wish you success.