I Want to Begin Learning (Began to learn) English Again

I am a graduate student, whose major was (from) Japanese Language and Culture.(major.) Though I had spent 7 years learning (to learn) English from middle school to high school, I didn't use those skills (used any English) during the (for) four years I was in university. As a result, I forgot most of the words I had learned/learnt (remembered) and when I want to speak in English, Japanese words come out into my mind instead. (stopping me from recalling English words.) 
Now, I have found a job which requires (needs) me to shift between these two languages fluently. It is not an easy thing (to do), but I want to take (use) this opportunity/chance to become adept (make myself good) at both languages. (of them.)
I have set myself a target which is to take (join) the TOEIC exam in August and achieve a satisfactory (get myself a satisfied) result. I'll try my best to succeed. (achieve it.) 

If anyone could correct my mistakes above, I would (will) be very very thankful~THX

I have suggested a few alternative phrases you might use to make your note a little smoother. Good luck with your studies!