I find really hard to express myself I mean it's pretty difficult write clearly what I mean. Any time I try to write something even for my job I am stuck, I look at the page and no words come to my mind and I feel lost I really don't know how to tell what I need to say. To be honest now this problem is becoming a real obstacle because I have to write email to customer of suppliers. My manager constantly corrects my email saying that they don't make sense and some times are too rude.
You can try to "rubber duck".
Explain what you need to convey out loud to someone not in the know or even to a rubber ducky from the bathtub. Then write it down.
When still unsure, wait a day and read it back to see if the text still makes sense.
Keep at it, talk about it, it is a learn-able skill!
There is a three part solution to your problem:
Format -- there are some standard forms for business communication. A lot of what makes any communication seem more polite is just the inclusion of some standard niceties (please, thank you, etc) that can basically be cut and pasted from one email to the next. You should easily be able to find some books or online tutorials for business communication. If you follow the pattern, all you need to do is adapt it to your situation --it doesn't need to be creative or inspired.
Rewrite -- never send an email until you have read it through a couple of times. It may help to read it out loud. It might also help you get started on your writing if you tell yourself that the first version doesn't need to be perfect, since you'll be rewriting later.
Practice -- no one is good at writing until they do a lot of it.
I feel a very good and simple way to start is a scratch pad. Hard to express is generally associated with difficulty in forming sentences which convey the intended thought. We always have the basic words ready, but the sentences is what the problem is.
Just write whatever comes in your mind. Words or sentences or pictures whatever it may be. First put all of it down somewhere.
Let these just be random dots for now. The next task is to join the dots. Just repeat this iteratively and form sentences. Finally you will get the intended. Initially you may have to do this if you are really having a hard time expressing but once you get a hang of the procedure. You will do away will at the putting down part.