Try this format
Dear Senator XXXXX XXXXXX,
I am writing this letter regarding an immigration issue, I came to the united states on DD-MM-YYYY as a professional worker on H1 Visa, My current employer XXXXXXXXXX were very happy with my performance and they started my Green Card process. Today I am in the third and last phase of my green card process, which is Application Type: I-485, Application to Register Permanent Residence or to Adjust Status. Case number EACXXXXXXXX
The I-485 processing time frame written on the BCIS receipt is from 365 to 540 days from the receipt date, in my case the receipt date is DD-MM-YYYY.
XX months have passed and I did not hear anything from BCIS, I submitted several inquiries through BCIS customer service and I did not hear anything from BCIS yet. By law I cannot do anything except waiting and I consumed the three inquiry attempts allowed by BCIS. BCIS Vermont Service Center GIT processing times records show BCIS are processing I-485 cases with receipt date DD-MM-YYYY. BCIS passed my I-485 case with no reply or referral. At this point I stand hopeless and desperate.
Dear Senator XXXXXX XXXXXXX , I desperately need your assistance and support, I did work for one employer since the day I came to the United States, I am a legal alien, I also obeyed the law and registered with BCIS registry program (registration number XXXXXXXX) and I have been a lawful resident with no records of crime or felony.
Dear Senator, I don’t know what’s the reason for this delay from BCIS, all what I can ask is for your support and assistance. I am sure I am not the only one who has gone through this delay.
I am attaching with this fax my legal residency status (Employment Authorization Card), my I-94 (Departure Number), I-140 and my I-485 receipt.
Thank you very much for taking your time reading my letter and I am filled with hope that you will help me in my unreasonable I-485 case delay.
Regards,