Who is to judge your worthiness? In this situation, it is the guide dog agency.
You were evaluated for a guide dog and found to be worthy. You met the requirements as determined by those whose job it is to make such a determination. If the evaluators did not believe that you were worthy, then you would have been turned down. In their eyes, you are deserving of your new companion, Jimbo.
It is their job to make that determination. You must accept their judgment. They judged you as being worthy of having a guide dog. By suggesting that you are unworthy, or not as worthy as someone else, you are in essence questioning their judgment.
Recognize that your judgment is biased. By suggesting that you are not worthy enough, you are devaluing yourself. Some people overvalue themselves and some undervalue themselves. Based on the tone and content of your letter, you seem to have a tendency to undervalue yourself. It is important to appropriately value yourself in all situations. In this situation, the agency has deemed you as worthy and as such you should accept their judgment.
The reality is you have been deemed worthy. That judgment should extend to all areas of your life. If you struggle with valuing yourself appropriately, then it would be beneficial to seek counseling. Individuals who consistently devalue themselves often struggle with self-esteem issues.
Thank you for updating me on your situation. I hope that you can see the wisdom of accepting reality, can appreciate your worthiness and will thoroughly enjoy your new companion. Please take care.