Dear Mary
I have (received) your E-mail just (then用“now"也可以).I'm getting on well with my research works in the lab(, but) to my surprise you say you will give up learn(ing) English. The reason is because (that) you have not done well in (the subject) recently, (so) you have lost interest. I'm afraid I couldn't agree with you. I know (it's) not easy to learn English. (这句话重整：However, English being a widely used language in the global world today,) it will be (an) important tool in (our 应该是指我们吧) future work. Besides, it's becoming (increasingly)important in our daily life. If you study hard, you will (succeed). Do remember that where there is a wall, there is (also) a way. I'm looking forward to hear good news from you.
(Yours sincerely
XX 你的名字
MM/DD/YEAR 日期)
Ps. 括号里的是已修改好的词或句